MB-11104 Set the open checkpoint id before merging checkpoints.

When we merge checkpoints in a replica vbucket, we should call
setOpenCheckpointId() first before merging checkpoints. Otherwise,
the byseq num of a checkpoint_start item in a collapsed checkpoint
won't be set correctly.

Change-Id: I5219da59c52f689fee1646f6d21aea1627596369
Reviewed-on: http://review.couchbase.org/37356
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>