Fix handling of collected results

The result collection did not reset properly when a non-resultset was
returned for a request. As collected result need to be distinguishable
from single packet responses, a new buffer type was added.

The new buffer type is used by readwritesplit which uses result collection
for preparation of prepared statements.

Moved the current command tracking to the RWBackend class as the command
tracked by the protocol is can change before a response to the executed
command is received.

Removed a false debug assertion in the mxs_mysql_extract_ps_response
function that was triggered when a very large prepared statement response
was processed in multiple parts.

server/modules/routing/readwritesplit/rwsplitsession.hh

@@ -69,12 +69,18 @@ public:
         return m_large_packet;
     }
 
+    inline uint8_t current_command() const
+    {
+        return m_command;
+    }
+
 private:
     reply_state_t    m_reply_state;
     BackendHandleMap m_ps_handles; /**< Internal ID to backend PS handle mapping */
     bool             m_large_packet; /**< Used to store the state of the EOF packet
                                       *calculation for result sets when the result
                                       * contains very large rows */
+    uint8_t          m_command;
 };
 
 typedef std::tr1::shared_ptr<RWBackend> SRWBackend;