Properly seek to checkpoint item when seqno is in middle of chk

This fixes a bug where when we needed to seek to some item in a
checkpoint when registering a cursor. The old code would always
put the cursor at the beginning of the checkpoint.

Change-Id: I432c42f2eac2bc45a67f459e8e23f9c0bd1e8214
Reviewed-on: http://review.couchbase.org/32627
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>