MB-11592: Create snapshots when getting items from the chk queues

This change introduces a new flag to indicate whether or not
ep-engine should create a new checkpoint for this snapshot. This
flag is used internally by ep-engine and only ep-engine. We are
also changing the flags values for snapshot markers since they
weren't really flags previously. Below are the new values.

Disk 0x01
Memory 0x02
Checkpoint 0x04

Change-Id: I8a262eb7856c97e11e95570fbdaef82f3f79054a
Reviewed-on: http://review.couchbase.org/38779
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>