Treat walproposer like walsenders in postmaster.

This keeps the walproposer processes alive at shutdown, until after
the shutdown checkpoint has been written. That gives the walproposers
a chance to stream it to the safekeepers.

Author: hlinnaka

src/backend/postmaster/postmaster.c

@@ -4136,7 +4136,8 @@ SignalSomeChildren(int signal, int target)
 			 * Assign bkend_type for any recently announced WAL Sender
 			 * processes.
 			 */
-			if (bp->bkend_type == BACKEND_TYPE_NORMAL &&
+			/* Neon: Also allow walproposer background worker to be treated like a WAL sender, so that it's shut down last */
+			if ((bp->bkend_type == BACKEND_TYPE_NORMAL || BACKEND_TYPE_BGWORKER) &&
 				IsPostmasterChildWalSender(bp->child_slot))
 				bp->bkend_type = BACKEND_TYPE_WALSND;
 
@@ -5416,7 +5417,8 @@ CountChildren(int target)
 			 * Assign bkend_type for any recently announced WAL Sender
 			 * processes.
 			 */
-			if (bp->bkend_type == BACKEND_TYPE_NORMAL &&
+			/* Neon: Also allow walproposer background worker to be treated like a WAL sender, so that it's shut down last */
+			if ((bp->bkend_type == BACKEND_TYPE_NORMAL || BACKEND_TYPE_BGWORKER) &&
 				IsPostmasterChildWalSender(bp->child_slot))
 				bp->bkend_type = BACKEND_TYPE_WALSND;