rowcache: change memcache_stats to poll

memcache_stats used to pull expvar values on demand. This introduced an
rpc dependency on expvar, which is not sound. With this change we
regularly pull stats from memcache and update the expvars.
Also, the new version has made the items & slabs stats optional, and
rowcache doesn't export them any more.
Merge branch 'rowcache'