Use latch instead of select() in walreceiver

Replace use of poll()/select() by WaitLatchOrSocket(), which is more
portable and flexible.

Also change walreceiver to use its procLatch instead of a custom latch.

From: Petr Jelinek <petr@2ndquadrant.com>

src/include/pgstat.h

@@ -763,6 +763,7 @@ typedef enum
 	WAIT_EVENT_CLIENT_WRITE,
 	WAIT_EVENT_SSL_OPEN_SERVER,
 	WAIT_EVENT_WAL_RECEIVER_WAIT_START,
+	WAIT_EVENT_LIBPQWALRECEIVER_READ,
 	WAIT_EVENT_WAL_SENDER_WAIT_WAL,
 	WAIT_EVENT_WAL_SENDER_WRITE_DATA
 } WaitEventClient;