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/replication/walreceiver.h

@@ -21,7 +21,6 @@
 #include "replication/logicalproto.h"
 #include "replication/walsender.h"
 #include "storage/condition_variable.h"
-#include "storage/latch.h"
 #include "storage/spin.h"
 #include "utils/tuplestore.h"
 
@@ -58,13 +57,24 @@ typedef enum
 typedef struct
 {
 	/*
-	 * PID of currently active walreceiver process, its current state and
-	 * start time (actually, the time at which it was requested to be
-	 * started).
+	 * Currently active walreceiver process's proc number and PID.
+	 *
+	 * The startup process uses the proc number to wake it up after telling it
+	 * where to start streaming (after setting receiveStart and
+	 * receiveStartTLI), and also to tell it to send apply feedback to the
+	 * primary whenever specially marked commit records are applied.
 	 */
+	ProcNumber	procno;
 	pid_t		pid;
+
+	/* Its current state */
 	WalRcvState walRcvState;
 	ConditionVariable walRcvStoppedCV;
+
+	/*
+	 * Its start time (actually, the time at which it was requested to be
+	 * started).
+	 */
 	pg_time_t	startTime;
 
 	/*
@@ -134,15 +144,6 @@ typedef struct
 	/* set true once conninfo is ready to display (obfuscated pwds etc) */
 	bool		ready_to_display;
 
-	/*
-	 * Latch used by startup process to wake up walreceiver after telling it
-	 * where to start streaming (after setting receiveStart and
-	 * receiveStartTLI), and also to tell it to send apply feedback to the
-	 * primary whenever specially marked commit records are applied. This is
-	 * normally mapped to procLatch when walreceiver is running.
-	 */
-	Latch	   *latch;
-
 	slock_t		mutex;			/* locks shared variables shown above */
 
 	/*