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 <daver@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>