Allow master changes at transaction start

When a BEGIN statement is being executed without a master connection but
when one can be created, the BEGIN statement would be treated as if a
transaction was already open. Since the statement only starts the
transaction, it is allowed to be routed to a "new" master regardless of
the transaction statem.

This fixes the failure to start a transaction when lazy_connect is
enabled.

server/modules/routing/readwritesplit/rwsplitsession.hh

@@ -166,6 +166,7 @@ private:
     bool            create_one_connection();
     void            retry_query(GWBUF* querybuf, int delay = 1);
 
+    bool trx_is_starting();
     bool should_replace_master(mxs::RWBackend* target);
     void replace_master(mxs::RWBackend* target);
     bool should_migrate_trx(mxs::RWBackend* target);