[BP] MB-29531: Replace revSeqno with a 48-bit counter

[BP] MB-29531: Replace revSeqno with a 48-bit counter

Prevent a value too large to be stored in couchstore
from being placed into Item/StoredValue and also the
_local document (via vbucket_state).

Change-Id: I6de783dc2374c2634f1a729e4ca5fa2bc35dda40
Reviewed-on: http://review.couchbase.org/93800
Well-Formed: Build Bot <build@couchbase.com>
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>