Add a test api to get 64 bit stat numbers

Not being able to get these numbers can cause crashes in
the testsuite since the values can be truncated.

Change-Id: Ic289144c637a840ea2816e0a6ab9e668804510d8
Reviewed-on: http://review.couchbase.org/31650
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>