Make Checkpoint::vbucketId const as it should never change

Change-Id: I621d297817430944d49131b54198b1fbc86e3dfa
Reviewed-on: https://review.couchbase.org/c/kv_engine/+/172734
Tested-by: Build Bot <[email protected]>
Reviewed-by: James H <[email protected]>

Author: rdemellow

engines/ep/src/checkpoint.h

@@ -649,7 +649,7 @@ class Checkpoint {
     uint64_t visibleSnapEndSeqno = 0;
     /// The seqno of the highest expelled item.
     Monotonic<int64_t> highestExpelledSeqno{0};
-    Vbid vbucketId;
+    const Vbid vbucketId;
     rel_time_t                     creationTime;
     folly::Synchronized<checkpoint_state> checkpointState;
     /// Number of non-meta items (see Item::isCheckPointMetaItem).