MXS-2350: Allow lazy connection creation

The lazy connection creation reduces the burden that short sessions place
on the backend servers. This also prevents the problems caused by early
disconnections that happen when only one server is used but multiple
connections are created. This does not solve the problem (MXS-619) but it
does mitigate it to acceptable levels.

This commit also adds a change to the weighting algorithm that prefers
existing connections over unopened ones. This helps avoid the
flip-flopping that happens when the absolute scores are very similar. The
hard-coded value might need to be tuned once testing is done.

server/modules/routing/readwritesplit/rwsplitsession.hh

@@ -133,6 +133,7 @@ public:
 private:
     RWSplitSession(RWSplit* instance,
                    MXS_SESSION* session,
+                   const Config& config,
                    mxs::SRWBackends backends,
                    mxs::RWBackend*  master);
 
@@ -290,7 +291,7 @@ private:
     uint64_t                m_sent_sescmd;      /**< ID of the last sent session command*/
     uint64_t                m_recv_sescmd;      /**< ID of the most recently completed session
                                                  * command */
-    ExecMap         m_exec_map;                 /**< Map of COM_STMT_EXECUTE statement IDs to
+    ExecMap m_exec_map;                         /**< Map of COM_STMT_EXECUTE statement IDs to
                                                  * Backends */
     std::string          m_gtid_pos;            /**< Gtid position for causal read */
     wait_gtid_state      m_wait_gtid;           /**< State of MASTER_GTID_WAIT reply */