"standby switchover": avoid potential race condition with WAL location check

Immediately after the demotion candidate (primary) has shut down, we can't
be absolutely sure that the walreceiver has flushed all WAL to disk, so
checking pg_last_wal_receive_lsn() at that point might not reflect
the actual last available WAL location.

To handle this, we'll loop for a while (timeout controlled by configuration
parameter "wal_receive_check_timeout") before finally deciding whether
the standby is still behind the shut-down primary.

Addresses issue raised in GitHub #518.

repmgr.conf.sample

@@ -241,6 +241,9 @@ ssh_options='-q -o ConnectTimeout=10'	# Options to append to "ssh"
 					# for the demoted standby to reconnect to the promoted
 					# primary (note: this value should be equal to or greater
 					# than that set for "node_rejoin_timeout")
+#wal_receive_check_timeout=30		# The max length of time (in seconds) to wait for the walreceiver
+					# on the standby to flush WAL to disk before comparing location
+					# with the shut-down primary
 
 #------------------------------------------------------------------------------
 # "node rejoin" settings