Add stats to indicate the snapshot a PassiveStream is receiving

These stats will help with debugging and will also be useful with
future commits that require information about the snapshot that is
currently being processed.

Change-Id: I2ed88b0e064ee7744c02d80a66b545b24d5e11a0
Reviewed-on: http://review.couchbase.org/39281
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>