Enable libevent state caching
Enable libevent state caching

When introducing the idle timer we introduced a "bug" that
disabled the optimization we've got to cache the libevent
state by caching.

Change-Id: Iea7f768f361c8ec3b9d9c59a5e439b5e2c0095f5
Reviewed-on: http://review.couchbase.org/57149
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Daniel Owen <b3fc0e1a51f252986e60be7da121b74d3ac6845f@couchbase.com>

daverigby/memcached,owendCB/memcached,daverigby/kv_engine,daverigby/memcached,couchbase/memcached,daverigby/kv_engine,daverigby/kv_engine,owendCB/memcached,couchbase/memcached,couchbase/memcached,daverigby/memcached,couchbase/memcached,owendCB/memcached,owendCB/memcached,daverigby/kv_engine,daverigby/memcached