Remove or reduce verbosity of some debug messages.

The debug messages that merely print StartTransactionCommand,
CommitTransactionCommand, ProcessUtilty, or ProcessQuery with no
additional details seem to be useless.  Get rid of them.

The transaction status messages produced by ShowTransactionState are
occasionally useful, but they are extremely verbose, producing
multiple lines of log output every time they fire, which can happens
multiple times per transaction.  So, reduce the level to DEBUG5; avoid
emitting an extra line just to explain which debug point is at issue;
and tighten up the rest of the message so it doesn't use quite so much
horizontal space.

With these changes, it's possible to run a somewhat busy system with a
log level even as high as DEBUG4, whereas previously anything above
DEBUG2 would flood the log with output that probably wasn't really all
that useful.

src/backend/tcop/postgres.c

@@ -2427,8 +2427,6 @@ start_xact_command(void)
 {
 	if (!xact_started)
 	{
-		ereport(DEBUG3,
-				(errmsg_internal("StartTransactionCommand")));
 		StartTransactionCommand();
 
 		/* Set statement timeout running, if any */
@@ -2450,10 +2448,6 @@ finish_xact_command(void)
 		/* Cancel any active statement timeout before committing */
 		disable_timeout(STATEMENT_TIMEOUT, false);
 
-		/* Now commit the command */
-		ereport(DEBUG3,
-				(errmsg_internal("CommitTransactionCommand")));
-
 		CommitTransactionCommand();
 
 #ifdef MEMORY_CONTEXT_CHECKING