Make it possible to reset couch_upr_fake_server

For testing it is needed to reset the state of the
couch_upr_fake_server.

Change-Id: I3f6d6b58b7a67bff970274dd6075adc67aad491d
Reviewed-on: http://review.couchbase.org/31297
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>