Remove uses of "slave" in replication contexts

This affects mostly code comments, some documentation, and tests.
Official APIs already used "standby".

src/backend/access/transam/commit_ts.c

@@ -621,7 +621,7 @@ CommitTsParameterChange(bool newvalue, bool oldvalue)
  *
  * The reason why this SLRU needs separate activation/deactivation functions is
  * that it can be enabled/disabled during start and the activation/deactivation
- * on master is propagated to slave via replay. Other SLRUs don't have this
+ * on master is propagated to standby via replay. Other SLRUs don't have this
  * property and they can be just initialized during normal startup.
  *
  * This is in charge of creating the currently active segment, if it's not

src/backend/access/transam/multixact.c

@@ -939,7 +939,7 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
 
 	debug_elog3(DEBUG2, "GetNew: for %d xids", nmembers);
 
-	/* safety check, we should never get this far in a HS slave */
+	/* safety check, we should never get this far in a HS standby */
 	if (RecoveryInProgress())
 		elog(ERROR, "cannot assign MultiXactIds during recovery");
 

src/backend/access/transam/varsup.c

@@ -67,7 +67,7 @@ GetNewTransactionId(bool isSubXact)
 		return BootstrapTransactionId;
 	}
 
-	/* safety check, we should never get this far in a HS slave */
+	/* safety check, we should never get this far in a HS standby */
 	if (RecoveryInProgress())
 		elog(ERROR, "cannot assign TransactionIds during recovery");
 
@@ -468,7 +468,7 @@ GetNewObjectId(void)
 {
 	Oid			result;
 
-	/* safety check, we should never get this far in a HS slave */
+	/* safety check, we should never get this far in a HS standby */
 	if (RecoveryInProgress())
 		elog(ERROR, "cannot assign OIDs during recovery");