MXS-1507: Add initial implementation of transaction replay

Added the initial implementation of transaction replay. Transactions are
only replayed if the master fails when no statement is being executed.

The validity of the replayed transaction is done by verifying that the
checksums of the returned results are equal.

Added a close function into the Trx class to make resetting its state
easier. Also changed the return type of the pop_stmt to GWBUF* as the
places where it is used expect a raw GWBUF pointer.

server/modules/routing/readwritesplit/rwsplitsession.hh

@@ -138,6 +138,8 @@ public:
     uint64_t                m_retry_duration; /**< Total time spent retrying queries */
     mxs::Buffer             m_current_query; /**< Current query being executed */
     Trx                     m_trx; /**< Current transaction */
+    bool                    m_is_replay_active; /**< Whether we are actively replaying a transaction */
+    Trx                     m_replayed_trx; /**< The transaction we are replaying */
 
 private:
     RWSplitSession(RWSplit* instance, MXS_SESSION* session,
@@ -181,8 +183,8 @@ private:
     void handle_error_reply_client(DCB *backend_dcb, GWBUF *errmsg);
     bool handle_error_new_connection(DCB *backend_dcb, GWBUF *errmsg);
 
-    // Currently only for diagnostic purposes
-    void close_transaction();
+    void handle_trx_replay();
+    void start_trx_replay();
 
 private:
     // QueryClassifier::Handler