Ignore recovery_min_apply_delay until recovery has reached consistent state

Previously recovery_min_apply_delay was applied even before recovery
had reached consistency. This could cause us to wait a long time
unexpectedly for read-only connections to be allowed. It's problematic
because the standby was useless during that wait time.

This patch changes recovery_min_apply_delay so that it's applied once
the database has reached the consistent state. That is, even if the delay
is set, the standby tries to replay WAL records as fast as possible until
it has reached consistency.

Author: Michael Paquier
Reviewed-By: Julien Rouhaud
Reported-By: Greg Clough
Backpatch: 9.4, where recovery_min_apply_delay was added
Bug: #13770
Discussion: http://www.postgresql.org/message-id/20151111155006.2644.84564@wrigleys.postgresql.org

doc/src/sgml/recovery-config.sgml

@@ -462,8 +462,9 @@ restore_command = 'copy "C:\\server\\archivedir\\%f" "%p"'  # Windows
         are not visible until the corresponding commit record is applied.
        </para>
        <para>
-        The delay occurs until the standby is promoted or triggered. After that
-        the standby will end recovery without further waiting.
+        The delay occurs once the database in recovery has reached a consistent
+        state, until the standby is promoted or triggered. After that the standby
+        will end recovery without further waiting.
        </para>
        <para>
         This parameter is intended for use with streaming replication deployments;

src/backend/access/transam/xlog.c

@@ -5715,6 +5715,10 @@ recoveryApplyDelay(XLogReaderState *record)
 	if (recovery_min_apply_delay <= 0)
 		return false;
 
+	/* no delay is applied on a database not yet consistent */
+	if (!reachedConsistency)
+		return false;
+
 	/*
 	 * Is it a COMMIT record?
 	 *