MXS-1507: Add transaction size limit

Added the `transaction_replay_max_size` parameter that controls the
maximum size of a transaction that can be replayed. If the limit is
exceeded, the stored statements are released thus preventing the
transaction from being replayed.

This limitation prevents accidental misuse of the transaction replaying
system when autocommit is disabled. It also allows the user to control the
amount of memory that MaxScale will use.

server/modules/routing/readwritesplit/readwritesplit.cc

@@ -507,6 +507,7 @@ MXS_MODULE *MXS_CREATE_MODULE()
             {"delayed_retry", MXS_MODULE_PARAM_BOOL, "false"},
             {"delayed_retry_timeout", MXS_MODULE_PARAM_COUNT, "10"},
             {"transaction_replay", MXS_MODULE_PARAM_BOOL, "false"},
+            {"transaction_replay_max_size", MXS_MODULE_PARAM_COUNT, "1Mi"},
             {MXS_END_MODULE_PARAMS}
         }
     };