pg: Rework txn implementation + statement config API

I liked the Perl implementation of transactions, but managing state
between Perl and C is a bit cumbersome, so I've moved the whole thing
into C.

Also added a few statement configuration methods that currently don't do
anything yet.

FU/PG.pm

@@ -6,61 +6,8 @@ _load_libpq();
 
 package FU::PG::conn {
     sub lib_version { FU::PG::lib_version() }
-
-    sub txn($c) {
-        $c->exec('BEGIN');
-        $c->_set_cookie(++$FU::PG::txn::COUNTER);
-        bless [$c, $FU::PG::txn::COUNTER, undef], 'FU::PG::txn';
-    }
 };
 
-package FU::PG::txn {
-    use Carp 'confess';
-
-    my $COUNTER = 0;
-
-    # Arrayref:
-    # 0: $conn
-    # 1: $cookie, a snapshot of $COUNTER that identifies this transaction, used
-    #    to match commands against transactions. Set to undef when this
-    #    transaction is 'done' but the object is still alive.
-    # 2: $parent, undef if this is a top-level transaction.
-
-    sub commit($t) {
-        confess "Unable to commit transaction that has already finished" if !$t->[1];
-        $t->exec($t->[2] ? "RELEASE SAVEPOINT fupg_$t->[1]" : 'COMMIT');
-        $t->[1] = undef;
-    }
-
-    sub rollback($t) {
-        confess "Unable to rollback transaction that has already finished" if !$t->[1];
-        $t->exec($t->[2] ? "ROLLBACK TO SAVEPOINT fupg_$t->[1]" : 'ROLLBACK');
-        $t->[1] = undef;
-    }
-
-    sub txn($t) {
-        confess "Unable to create sub-transaction when current transaction has already finished" if !$t->[1];
-        $COUNTER++;
-        $t->exec("SAVEPOINT fupg_$COUNTER");
-        $t->[0]->_set_cookie($COUNTER);
-        bless [$t->[0], $COUNTER, $t], 'FU::PG::txn';
-    }
-
-    sub status($t) {
-        my $cs = $t->[0]->status;
-        return $cs if $cs eq 'bad' || ($t->[1] && $t->[0]->_get_cookie != $t->[1]);
-        return $cs eq 'txn_error' ? 'error' : $t->[1] ? 'idle' : 'done';
-    }
-
-    sub DESTROY($t) {
-        # Can't really throw an error in DESTROY. If a rollback command fails,
-        # we're sufficiently screwed that the only sensible recourse is to
-        # disconnect and let any further operations throw an error.
-        eval { $t->rollback; 1 } || $t->[0]->disconnect if $t->[1];
-        $t->[0]->_set_cookie($t->[2] ? $t->[2][1] : 0);
-    }
-}
-
 package FU::PG::error {
     use overload '""' => sub($e, @) { $e->{full_message} };
 }
@@ -163,6 +110,16 @@ Connection is dead or otherwise unusable.
 
 =back
 
+=item B<< $conn->cache($enable) >>
+
+=item B<< $conn->text_params($enable) >>
+
+=item B<< $conn->text_results($enable) >>
+
+=item B<< $conn->text($enable) >>
+
+Set the default settings for new statements created with B<< $conn->q() >>.
+
 =item B<< $conn->disconnect >>
 
 Close the connection. Any active transactions are rolled back and any further
@@ -198,8 +155,30 @@ used.
 
 =back
 
-Statement objects returned by C<< $conn->q() >> can be inspected with the
-following two methods:
+Statement objects returned by C<< $conn->q() >> support the following
+configuration parameters:
+
+=over
+
+=item B<< $st->cache($enable) >>
+
+Enable or disable caching of the prepared statement for this particular query.
+
+=item B<< $st->text_params($enable) >>
+
+Enable or disable sending bind parameters in the text format.
+
+=item B<< $st->text_results($enable) >>
+
+Enable or disable receiving query results in the text format.
+
+=item B<< $st->text($enable) >>
+
+Shorthand for setting C<text_params> and C<text_results> at the same time.
+
+=back
+
+Statement objects can be inspected with the following two methods:
 
 =over
 
@@ -264,6 +243,13 @@ multiple columns with the same name.
 
 =back
 
+The only time you actually need to assign a statement object to a variable is
+when you want to inspect C<params> or C<columns>, in all other cases you can
+chain the methods for more concise code. For example:
+
+  my @cols = $conn->q('SELECT a, b FROM table')->cache(0)->text->rowa;
+
+
 =head2 Transactions
 
 This module provides a convenient and safe API for I<scoped transactions> and
@@ -298,12 +284,16 @@ Transaction methods:
 
 =over
 
-=item B<< $txn->exec(..) >> and B<< $txn->q(..) >>
+=item B<< $txn->exec(..) >>
+
+=item B<< $txn->q(..) >>
 
 Run a query inside the transaction. These work the same as the respective
 methods on the parent C<$conn> object.
 
-=item B<< $txn->commit >> and B<< $txn->rollback >>
+=item B<< $txn->commit >>
+
+=item B<< $txn->rollback >>
 
 Commit or abort the transaction. Any attempts to run queries on this
 transaction object after this call will throw an error.
@@ -311,6 +301,21 @@ transaction object after this call will throw an error.
 Calling C<rollback> is optional, the transaction is automatically rolled back
 when the object goes out of scope.
 
+=item B<< $txn->cache($enable) >>
+
+=item B<< $txn->text_params($enable) >>
+
+=item B<< $txn->text_results($enable) >>
+
+=item B<< $txn->text($enable) >>
+
+Set the default settings for new statements created with B<< $txn->q() >>.
+
+These settings are inherited from the main connection when the transaction is
+created. Subtransactions inherit these settings from their parent transaction.
+Changing these settings within a transaction does not affect the main
+connection or any already existing subtransactions.
+
 =item B<< $txn->txn >>
 
 Create a subtransaction within the current transaction. A subtransaction works
@@ -386,7 +391,7 @@ your database encoding is UTF-8. Non-UTF-8 databases are still supported with
 the text format by setting `client_encoding=utf8` as part of the connection
 string or by manually switching to it after C<connect()>:
 
-  my $conn = FU::PG->connect("");
+  my $conn = FU::PG->connect("")->text;
   $conn->exec('SET client_encoding=utf8');
 
 (But you're missing out on most features this module has to offer if you're