Fix RWBackend state tracking with multi-result queries

If a query returned multiple resultsets and the connection was broken
between the resultsets, the backend would not know that parts of the
response were already sent. This is caused by the cyclic nature of the
state machine when multi-result responses are being processed.

To fix the problem, the result size is tracked to know how many bytes have
been sent to the client. This is a backport of the
MySQLProtocol::Result::size from 2.5(develop).

include/maxscale/protocol/rwbackend.hh

@@ -240,7 +240,7 @@ public:
      */
     bool reply_has_started() const
     {
-        return m_reply_state != REPLY_STATE_START && m_reply_state != REPLY_STATE_DONE;
+        return m_size > 0 && m_reply_state != REPLY_STATE_DONE;
     }
 
     void process_packets(GWBUF* buffer);
@@ -269,6 +269,7 @@ private:
     bool             m_large_query = false;
     bool             m_skip_next = false;
     Error            m_error;
+    uint64_t         m_size = 0;/**< Size of the response */
 
     /**
      * @param it   Iterator pointing to the command byte of an error packet.