Replace walsender's latch with the general shared latch.

Relying on the normal shared latch simplifies interrupt/signal
handling because we can rely on all signal handlers setting the proc
latch. That in turn allows us to avoid the use of
ImmediateInterruptOK, which arguably isn't correct because
WaitLatchOrSocket isn't declared to be immediately interruptible.

Also change sections that wait on the walsender's latch to notice
interrupts quicker/more reliably and make them more consistent with
each other.

This is part of a larger "get rid of ImmediateInterruptOK" series.

Discussion: 20150115020335.GZ5245@awork2.anarazel.de

src/include/replication/walsender_private.h

@@ -51,10 +51,10 @@ typedef struct WalSnd
 	slock_t		mutex;
 
 	/*
-	 * Latch used by backends to wake up this walsender when it has work to
-	 * do.
+	 * Pointer to the walsender's latch. Used by backends to wake up this
+	 * walsender when it has work to do. NULL if the walsender isn't active.
 	 */
-	Latch		latch;
+	Latch		*latch;
 
 	/*
 	 * The priority order of the standby managed by this WALSender, as listed