Couch-KVStore does not validate parsing JSON values from ::readVBState

From the snippet from CouchKVStore::readVBState, we call parseUint64 which internally will only update the output if the input is valid, and return false otherwise. We should check the return value from these function calls and handle them appropriately.

state = VBucket::fromString(vb_state.c_str());

parseUint64(max_deleted_seqno, &maxDeletedSeqno);

parseUint64(checkpoint_id, &checkpointId);