Fix pq_putmessage_noblock() to not block.

An evident copy-and-pasteo in commit 2bd9e412 broke the non-blocking
aspect of pq_putmessage_noblock(), causing it to behave identically to
pq_putmessage().  That function is nowadays used only in walsender.c,
so that the net effect was to cause walsenders to hang up waiting for
the receiver in situations where they should not.

Kyotaro Horiguchi

Patch: <20160728.185228.58375982.horiguchi.kyotaro@lab.ntt.co.jp>

src/include/libpq/libpq.h

@@ -43,7 +43,7 @@ extern PGDLLIMPORT PQcommMethods *PqCommMethods;
 #define pq_putmessage(msgtype, s, len) \
 	(PqCommMethods->putmessage(msgtype, s, len))
 #define pq_putmessage_noblock(msgtype, s, len) \
-	(PqCommMethods->putmessage(msgtype, s, len))
+	(PqCommMethods->putmessage_noblock(msgtype, s, len))
 #define pq_startcopyout() (PqCommMethods->startcopyout())
 #define pq_endcopyout(errorAbort) (PqCommMethods->endcopyout(errorAbort))