Change session id to 64bit

The server internal session id may be larger than 4 bytes (MariaDB uses 8)
but only 4 are sent in the handshake. The full value can be queried
from the server, but this query is not supported by MaxScale yet. In any
case, both the protocol and MXS_SESSION now have 64 bit counters. Only the
low 32 bits are sent in the handshake, similar to server.

include/maxscale/session.h

@@ -134,7 +134,7 @@ typedef struct session
 {
     skygw_chk_t             ses_chk_top;
     mxs_session_state_t     state;            /*< Current descriptor state */
-    uint32_t                ses_id;           /*< Unique session identifier */
+    uint64_t                ses_id;           /*< Unique session identifier */
     struct dcb              *client_dcb;      /*< The client connection */
     struct mxs_router_session *router_session;  /*< The router instance data */
     MXS_SESSION_STATS       stats;            /*< Session statistics */
@@ -194,7 +194,7 @@ MXS_SESSION *session_alloc(struct service *, struct dcb *);
  * @param id            Id for the new session.
  * @return              The newly created session or NULL if an error occurred
  */
-MXS_SESSION *session_alloc_with_id(struct service *, struct dcb *, uint32_t);
+MXS_SESSION *session_alloc_with_id(struct service *, struct dcb *, uint64_t);
 
 MXS_SESSION *session_set_dummy(struct dcb *);
 
@@ -352,14 +352,14 @@ static inline bool session_set_autocommit(MXS_SESSION* ses, bool autocommit)
  *
  * @note The caller must free the session reference by calling session_put_ref
  */
-MXS_SESSION* session_get_by_id(uint32_t id);
+MXS_SESSION* session_get_by_id(uint64_t id);
 
 /**
  * Get the next available unique (assuming no overflow) session id number.
  *
  * @return An unused session id.
  */
-uint32_t session_get_next_id();
+uint64_t session_get_next_id();
 
 /**
  * @brief Close a session