Use ProcNumbers instead of direct Latch pointers to address other procs

This is in preparation for replacing Latches with a new abstraction.
That's still work in progress, but this seems a little tidier anyway,
so let's get this refactoring out of the way already.

Discussion: https://www.postgresql.org/message-id/391abe21-413e-4d91-a650-b663af49500c%40iki.fi

src/include/access/xlogwait.h

@@ -15,7 +15,7 @@
 #include "lib/pairingheap.h"
 #include "postgres.h"
 #include "port/atomics.h"
-#include "storage/latch.h"
+#include "storage/procnumber.h"
 #include "storage/spin.h"
 #include "tcop/dest.h"
 
@@ -29,11 +29,8 @@ typedef struct WaitLSNProcInfo
 	/* LSN, which this process is waiting for */
 	XLogRecPtr	waitLSN;
 
-	/*
-	 * A pointer to the latch, which should be set once the waitLSN is
-	 * replayed.
-	 */
-	Latch	   *latch;
+	/* Process to wake up once the waitLSN is replayed */
+	ProcNumber	procno;
 
 	/* A pairing heap node for participation in waitLSNState->waitersHeap */
 	pairingheap_node phNode;