pg: Add query tracing & prepare/execute time measurements

What I'd really like, in addition to this, is a way to extract a query
from an $st object that can be run in the psql CLI. VNDB has a debugging
feature for that, but it's less trivial to make that work with binary
query parameters.

c/pgconn.c

@@ -29,6 +29,7 @@ static void fupg_prep_destroy(fupg_prep *p) {
 typedef struct {
     SV *self;
     PGconn *conn;
+    SV *trace;
     UV prep_counter;
     UV cookie_counter;
     UV cookie; /* currently active transaction object; 0 = none active */
@@ -159,6 +160,7 @@ static SV *fupg_connect(pTHX_ const char *str) {
 
     fupg_conn *c = safemalloc(sizeof(fupg_conn));
     c->conn = conn;
+    c->trace = NULL;
     c->prep_counter = c->cookie_counter = c->cookie = 0;
     c->stflags = FUPG_CACHE;
     c->ntypes = 0;