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