Make crash recovery ignore recovery_min_apply_delay setting.

In v11 or before, this setting could not take effect in crash recovery
because it's specified in recovery.conf and crash recovery always
starts without recovery.conf. But commit 2dedf4d9 integrated
recovery.conf into postgresql.conf and which unexpectedly allowed
this setting to take effect even in crash recovery. This is definitely
not good behavior.

To fix the issue, this commit makes crash recovery always ignore
recovery_min_apply_delay setting.

Back-patch to v12 where the issue was added.

Author: Fujii Masao
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/CAHGQGwEyD6HdZLfdWc+95g=VQFPR4zQL4n+yHxQgGEGjaSVheQ@mail.gmail.com
Discussion: https://postgr.es/m/e445616d-023e-a268-8aa1-67b8b335340c@pgmasters.net

doc/src/sgml/config.sgml

@@ -4209,7 +4209,8 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
        </para>
        <para>
         This parameter is intended for use with streaming replication deployments;
-        however, if the parameter is specified it will be honored in all cases.
+        however, if the parameter is specified it will be honored in all cases
+        except crash recovery.
 
         <varname>hot_standby_feedback</varname> will be delayed by use of this feature
         which could lead to bloat on the master; use both together with care.

src/backend/access/transam/xlog.c

@@ -5977,6 +5977,10 @@ recoveryApplyDelay(XLogReaderState *record)
 	if (!reachedConsistency)
 		return false;
 
+	/* nothing to do if crash recovery is requested */
+	if (!ArchiveRecoveryRequested)
+		return false;
+
 	/*
 	 * Is it a COMMIT record?
 	 *