MB-12673: Check for new checkpoint items after takeoverSend

Check for any new checkpoint items when the readyQ is found to
be empty in the takeoverSendPhase, before setting the old
vbucket to dead and the new vbucket to active.

Change-Id: I2a6ddacc711f5db42a1e3c575ae18d0b2b3126bd
Reviewed-on: http://review.couchbase.org/43918
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>