Investigate implementing EventuallyPersistentEngine::deleteVBucket in terms of KVBucket::setVBucketState

The current path for DEL_VBUCKET commands hits the function EventuallyPersistentEngine::deleteVBucket which is implemented partially in KVBucket::deleteVBucket. The general path for changing a vBucket state is in KVBucket::setVBucketState however and it is easy to forget that vBuckets can change state (to dead) via the DEL_VBUCKET path when adding to the code there. Investigate implementing the DEL_VBUCKET path in terms of KVBucket::setVBucketState so that all vBucket state changes are implemented in one place in the code.