Clean up instance and session lifecycle functions

The createInstace, newSession, closeSession and freeSession functions were
cleaned up and reorganized to be a bit clearer for the reader. Removed
unnecessary comments and replaced them with ones that explain what's
happening in the code.

Removed unused linked lists from both sessions and instances and replaced
them with better alternatives. This should improve performance since new
session don't have to acquire the instance level lock to put themselves
into the session list.

server/modules/routing/readwritesplit/readwritesplit.h

@@ -304,12 +304,10 @@ typedef struct
 typedef struct router_instance
 {
     SERVICE*                service;     /*< Pointer to service */
-    ROUTER_CLIENT_SES*      connections; /*< List of client connections */
     SPINLOCK                lock;        /*< Lock for the instance data */
     rwsplit_config_t        rwsplit_config; /*< expanded config info from SERVICE */
     int                     rwsplit_version; /*< version number for router's config */
     ROUTER_STATS            stats;       /*< Statistics for this router */
-    struct router_instance* next;        /*< Next router on the list */
     bool                    available_slaves; /*< The router has some slaves avialable */
 } ROUTER_INSTANCE;