jemalloc: Ensure stats are up-to-date
jemalloc: Ensure stats are up-to-date

jemalloc can cache it's statistics. Add a call to mallctl("epoch")
when fetching allocator stats to ensure that we have up-to-date stats
values.

Change-Id: Ie466e193e442a8b9dbb75367c3cc44e4cdbef90f
Reviewed-on: http://review.couchbase.org/39996
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>
Tested-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>

cloudrain21/memcached-1,owendCB/memcached,mrkwse/memcached,jimwwalker/memcached,daverigby/memcached,daverigby/memcached,owendCB/memcached,daverigby/kv_engine,couchbase/memcached,mrkwse/memcached,couchbase/memcached,daverigby/memcached,mrkwse/memcached,jimwwalker/memcached,owendCB/memcached,couchbase/memcached,cloudrain21/memcached-1,daverigby/kv_engine,cloudrain21/memcached-1,daverigby/kv_engine,daverigby/kv_engine,mrkwse/memcached,owendCB/memcached,daverigby/memcached,couchbase/memcached,jimwwalker/memcached,cloudrain21/memcached-1