MXS-1823 Replace meaningless eventq info with meaningful

The evq_length file held the returned number of descriptors from
the last epoll_wait() call. As such it is highly temporal and not
particularly meaningful.

That has now been removed and the instead the average number of
returned descriptors is maintained. That information changes slowly
and thus carries some meaning.

include/maxscale/poll.h

@@ -33,7 +33,7 @@ typedef enum
     POLL_STAT_ERROR,
     POLL_STAT_HANGUP,
     POLL_STAT_ACCEPT,
-    POLL_STAT_EVQ_LEN,
+    POLL_STAT_EVQ_AVG,
     POLL_STAT_EVQ_MAX,
     POLL_STAT_MAX_QTIME,
     POLL_STAT_MAX_EXECTIME

include/maxscale/worker.hh

@@ -50,7 +50,7 @@ struct WORKER_STATISTICS
     int64_t  n_pollev;                    /*< Number of polls returning events */
     int64_t  n_nbpollev;                  /*< Number of polls returning events */
     int64_t  n_fds[MAXNFDS];              /*< Number of wakeups with particular n_fds value */
-    int64_t  evq_length;                  /*< Event queue length */
+    int64_t  evq_avg;                     /*< Average event queue length */
     int64_t  evq_max;                     /*< Maximum event queue length */
     int64_t  blockingpolls;               /*< Number of epoll_waits with a timeout specified */
     uint32_t qtimes[N_QUEUE_TIMES + 1];