Add a default local latch for use in signal handlers.

To do so, move InitializeLatchSupport() into the new common process
initialization functions, and add a new global variable MyLatch.

MyLatch is usable as soon InitPostmasterChild() has been called
(i.e. very early during startup). Initially it points to a process
local latch that exists in all processes. InitProcess/InitAuxiliaryProcess
then replaces that local latch with PGPROC->procLatch. During shutdown
the reverse happens.

This is primarily advantageous for two reasons: For one it simplifies
dealing with the shared process latch, especially in signal handlers,
because instead of having to check for MyProc, MyLatch can be used
unconditionally. For another, a later patch that makes FEs/BE
communication use latches, now can rely on the existence of a latch,
even before having gone through InitProcess.

Discussion: 20140927191243.GD5423@alap3.anarazel.de

src/backend/utils/adt/misc.c

@@ -413,10 +413,10 @@ pg_sleep(PG_FUNCTION_ARGS)
 		else
 			break;
 
-		(void) WaitLatch(&MyProc->procLatch,
+		(void) WaitLatch(MyLatch,
 						 WL_LATCH_SET | WL_TIMEOUT,
 						 delay_ms);
-		ResetLatch(&MyProc->procLatch);
+		ResetLatch(MyLatch);
 	}
 
 	PG_RETURN_VOID();