Several cleanup items: - Refactor anonymous function into named function `accumulate_if_user_matches_metadata` - use `default` and `resolver` options to pick LWW value in   metadata object, and remove ?TOMBSTONE function head by returning   an empty list for Options.
Several cleanup items:
- Refactor anonymous function into named function `accumulate_if_user_matches_metadata`
- use `default` and `resolver` options to pick LWW value in
  metadata object, and remove ?TOMBSTONE function head by returning
  an empty list for Options.
Kyorai/riak_core