Rewrite the validator in ES6/Promises, several improvements here:

- implements a validation work queue, jobs are run during browser idle callbacks
- when merging base entities, don't run validations 2x on both base and head graphs (this was wasteful)
- keep track of resolved issues in a separate set (it's not a simple compare of base/head anymore)
  this happens after validation queue is empty and avoids race conditions and inaccurate resolved counts