Set the current byseqno in the checkpoint manager for backfilled items

If we don't do this then it can cause a sequence number to be re-used
once new items arrive after a backfill.

Change-Id: I2bdcde7bf64c4280ea1457e7ab2c6572d07132c0
Reviewed-on: http://review.couchbase.org/37350
Reviewed-by: Sriram Ganesan <sriram@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>