MB-6368: Add force shutdown variable to stats.hh

This change is needed in order to reduce code dependencies in
couch kvstore. By reducing dependencies it will allow us to
write unit tests for couch kvstore and will also reduce
dependencies for multi-reader/writer changes.

Change-Id: I0c80938b8a3b5427b7a1c6ba0f8aea1bc8cf0475
Reviewed-on: http://review.couchbase.org/25478
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>