MB-12633: Don't sleep right after scheduling a backfill manager task

Change-Id: I97dd4a91b036e4334f8338b6a6831ac959ac5773
Reviewed-on: http://review.couchbase.org/43103
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>
MB-12540: Refactor the way we track snapshot start/end seqnos

For the 3.0 release I needed to add snapshot sequence number
quickly and in doing so implemented an approach that was not
very good and prone to errors. The correct approach is to track
sequence numbers on disk and also to track them for each checkpoint
in the checkpoint manager. This change moves us to the better
approach.

Change-Id: Ifbdd6c87b06767ff8c6504c1058ce11195fa2428
WIP: Initial code to allow replica streams while receiving backfill

Change-Id: I21f57a071fd45ee59d63894f79e7c1a2fd2d5f25