Make GUC wal_receiver_timeout user-settable.

When multiple subscribers connect to different publisher servers,
it can be useful to set different wal_receiver_timeout values for
each connection to better detect failures. However, previously
this wasn't possible, which limited flexibility in managing subscriptions.

This commit changes wal_receiver_timeout to be user-settable,
allowing different values to be assigned using ALTER ROLE SET for
each subscription owner. This effectively enables per-subscription
configuration.

Author: MasaoFujii

doc/src/sgml/config.sgml

@@ -5147,9 +5147,6 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
         If this value is specified without units, it is taken as milliseconds.
         The default value is 60 seconds.
         A value of zero disables the timeout mechanism.
-        This parameter can only be set in
-        the <filename>postgresql.conf</filename> file or on the server
-        command line.
        </para>
       </listitem>
      </varlistentry>

src/backend/utils/misc/guc_tables.c

@@ -2308,7 +2308,7 @@ struct config_int ConfigureNamesInt[] =
 	},
 
 	{
-		{"wal_receiver_timeout", PGC_SIGHUP, REPLICATION_STANDBY,
+		{"wal_receiver_timeout", PGC_USERSET, REPLICATION_STANDBY,
 			gettext_noop("Sets the maximum wait time to receive data from the sending server."),
 			gettext_noop("0 disables the timeout."),
 			GUC_UNIT_MS