Read current vb state only if local doc exists
Read current vb state only if local doc exists

When setting new vbucket state of a vbucket, read exisiting
vbucket state only if the database file for the vbucket exists

Change-Id: I3cfef8f353c42c0adcfe669d47307f6f5fd0f1d5
Reviewed-on: http://review.couchbase.org/15689
Tested-by: Jin Lim <c45cd80a556f432a3607171a6818cd5a074b5cd4@couchbase.com>
Reviewed-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>

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