Create thread specific epoll instances

Having a unique epoll instance for each thread allows a lot of the locking
from poll.c to be removed. The downside to this is that each session can
have only one thread processing events for it which might reduce
performance with very low client counts.

include/maxscale/dcb.h

@@ -227,6 +227,7 @@ typedef struct dcb
     DCBEVENTQ       evq;            /**< The event queue for this DCB */
     int             fd;             /**< The descriptor */
     dcb_state_t     state;          /**< Current descriptor state */
+    int             owner;          /**< Owning thread */
     SSL_STATE       ssl_state;      /**< Current state of SSL if in use */
     int             flags;          /**< DCB flags */
     char            *remote;        /**< Address of remote end */