Move decrCursor/Pos out of the CheckpointManager

These functions do not require any state from the CheckpointManager
and should be moved to the CheckpointCursor class.

Change-Id: I9b924b120fde85ce9ecafbee131ab6bf9006e49a
Reviewed-on: http://review.couchbase.org/43071
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>