CheckpointManager: Make vbucketId const
CheckpointManager: Make vbucketId const

It's not possible to change the vBucketID a CheckpointManager is
responsible for.

Change-Id: Iece3dc7d736797652c074392b817e184765f6846
Reviewed-on: http://review.couchbase.org/59872
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>

couchbase/ep-engine,membase/ep-engine,teligent-ru/ep-engine,hisundar/ep-engine,couchbase/ep-engine,owendCB/ep-engine,membase/ep-engine,owendCB/ep-engine,membase/ep-engine,daverigby/kv_engine,daverigby/kv_engine,hisundar/ep-engine,teligent-ru/ep-engine,teligent-ru/ep-engine,daverigby/kv_engine,daverigby/ep-engine,owendCB/ep-engine,couchbase/ep-engine,hisundar/ep-engine,daverigby/ep-engine,hisundar/ep-engine,owendCB/ep-engine,daverigby/kv_engine,membase/ep-engine,teligent-ru/ep-engine,daverigby/ep-engine,couchbase/ep-engine,daverigby/ep-engine