MB-16181: Make ActiveStream track the current collections separator

MB-16181: Make ActiveStream track the current collections separator

This patch updates the ActiveStream so that it stores a copy of the current
separator and tracks changes to as they are transmitted through the checkpoint
via the separator changed SystemEvent.

Change-Id: Ie3ea87d006b0bbab3e0edd8895a4756c7c5d9fe8
Reviewed-on: http://review.couchbase.org/75502
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Build Bot <build@couchbase.com>