MXS-1804: Allow large session commands

Session commands that span multiple packets are now allowed and will
work. However, if one is executed the session command history is disabled
as no interface for appending to session commands exists.

The backend protocol modules now also correctly track the current
command. This was a pre-requisite for large session commands as they
needed to be gathered into a single buffer and to do this the current
command had to be accurate.

Updated tests to expect success instead of failure for large prepared
statements.

include/maxscale/protocol/mysql.h

@@ -355,6 +355,7 @@ typedef struct
     bool                   collect_result;               /*< Collect the next result set as one buffer */
     bool                   changing_user;
     uint32_t               num_eof_packets;              /*< Encountered eof packet number, used for check packet type */
+    bool                   large_query;                  /*< Whether to ignore the command byte of the next packet*/
 #if defined(SS_DEBUG)
     skygw_chk_t            protocol_chk_tail;
 #endif