Add WalRcvGetState() to retrieve the state of a WAL receiver

This has come up as useful as an alternative of WalRcvStreaming(), to be
able to do sanity checks based on the state of a WAL receiver.  This
will be used in a follow-up commit.

Author: Xuneng Zhou <xunengzhou@gmail.com>
Discussion: https://postgr.es/m/19093-c4fff49a608f82a0@postgresql.org

src/backend/replication/walreceiverfuncs.c

@@ -119,6 +119,20 @@ WalRcvRunning(void)
 		return false;
 }
 
+/* Return the state of the walreceiver. */
+WalRcvState
+WalRcvGetState(void)
+{
+	WalRcvData *walrcv = WalRcv;
+	WalRcvState state;
+
+	SpinLockAcquire(&walrcv->mutex);
+	state = walrcv->walRcvState;
+	SpinLockRelease(&walrcv->mutex);
+
+	return state;
+}
+
 /*
  * Is walreceiver running and streaming (or at least attempting to connect,
  * or starting up)?