Replace the plain GWBUF query queue with std::deque

Using a std::deque to store the queries retains the exact state of the
object thus removing the need to parse the query again. It also removes
the need to split the queue into individual packets which makes the code
cleaner.

server/modules/routing/readwritesplit/rwsplitsession.hh

@@ -16,6 +16,7 @@
 #include "trx.hh"
 
 #include <string>
+#include <deque>
 
 #include <maxscale/buffer.hh>
 #include <maxscale/modutil.hh>
@@ -269,7 +270,7 @@ private:
 
     inline bool can_route_queries() const
     {
-        return m_query_queue == NULL
+        return m_query_queue.empty()
                && (m_expected_responses == 0
                    || m_qc.load_data_state() == mxs::QueryClassifier::LOAD_DATA_ACTIVE
                    || m_qc.large_query());
@@ -319,7 +320,7 @@ private:
     uint64_t         m_sescmd_count;            /**< Number of executed session commands (starts from 1) */
     int              m_expected_responses;      /**< Number of expected responses to the current
                                                  * query */
-    GWBUF*                  m_query_queue;      /**< Queued commands waiting to be executed */
+    std::deque<GWBUF*>      m_query_queue;      /**< Queued commands waiting to be executed */
     RWSplit*                m_router;           /**< The router instance */
     mxs::SessionCommandList m_sescmd_list;      /**< List of executed session commands */
     ResponseMap             m_sescmd_responses; /**< Response to each session command */