session.c: Replaced free(session->router_sesision) with call to freeSession callback
users.c: Removed reference to uninitialized variable.
router.h: Added freeSession callback to function block.
mysql_backend.c: try to ensure that client dcb is still listening in epoll_wait when writing reply to it.
mysql_common.c: assert debug build is mysql_protocol_init is called with dcb == NULL
readconnroute.c, readwritesplit.c, debugcli.c and testroute.c : Added freeSession to function block and an inmplementation of it.
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