* If a DB_REPLICA connection has been opened already, then wait immediately.
* Otherwise sets a variable telling it to wait if such a connection is opened.
*
+ * This only applies to connections to the generic replica DB for this request.
+ * If a timeout happens when waiting, then getLaggedReplicaMode()/laggedReplicaUsed()
+ * will return true.
+ *
* @param DBMasterPos|bool $pos Master position or false
*/
public function waitFor( $pos );