dcb.c, gateway.c little tuning.

poll.c
	Removed mutex from epoll_wait.
	Removed read and write mutexes from poll_waitevents.

session.c
	If session_alloc fails, instead of calling directly free(session), call session_free, which decreases refcounter and only frees session when there are no references left. 
	Added session_unlink_dcb function which removes link from session and optionally sets the dcb->session pointer to NULL.

readconnection.h, readwritesplit.h
	Added check fields to ROUTER_CLIENT_SES strct as well as lock, version number (not used yet) and closed flag.

mysql_backend.c
	gw_read_backend_event: if backend_protocol->state was set to MYSQL_AUTH_RECV, function returned, which was unnecessary. If mysql state became MYSQL_AUTH_FAILED, router client session was closed. Removed unnecessary NULL checks because rsession is not allowed to be NULL. Similarly, removed other NULL checks and replaced them with asserts checking that router client session is not NULL at any phase.

mysql_client.c
	Removed unused code blocks. Polished log commands. Replaced router client sessions NULL checks with asserts.

mysql_common.c
	mysql_send_custom_error: if called with dcb == NULL, return.

readconnroute.c
	Replaced malloc with calloc. Added functions rses_begin_router_action and rses_exit_router_action. If router client session is not closed, they take a lock and release it, respectively. Those functions are used for protecting all operations which modify the contents of router client session struct.

readwritesplit.c
	Identical changes than in readconnroute.c

skygw_debug.h
	Added check number and - macro for ROUTER_CLIENT_SES, and added COM_QUIT to STRPACKETTYPE.

server/modules/include/readconnection.h

@@ -45,10 +45,19 @@ typedef struct backend {
  * The client session structure used within this router.
  */
 typedef struct router_client_session {
+#if defined(SS_DEBUG)
+        skygw_chk_t     rses_chk_top;
+#endif
+        SPINLOCK        rses_lock;     /**< protects rses_deleted */
+        int             rses_versno;   /**< even = no active update, else odd */
+        bool            rses_closed;   /**< true when closeSession is called */
 	BACKEND		*backend;	/**< Backend used by the client session */
 	DCB		*backend_dcb;	/**< DCB Connection to the backend */
 	struct router_client_session 
 			*next;
+#if defined(SS_DEBUG)
+        skygw_chk_t     rses_chk_tail;
+#endif
 } ROUTER_CLIENT_SES;
 
 /**