Add an API that lets consumers wait on language service to be up-to-date with a given project version

[davkean]

We need an API that lets consumers wait on the language service to be up-to-date with a given project version, similar to IProjectTreeService.PublishTree:

Task PublishLatestAsync(
  CancellationToken cancellationToken = default(CancellationToken));

Task PublishAsync(
  IImmutableDictionary<NamedIdentity, IProjectVersionRequirement> minimumRequiredDataSourceVersions,
  CancellationToken cancellationToken = default(CancellationToken));

This lets services such as CodeModel provider to block this before returning a CodeModel item, or the renamer to block on this attempting to rename. Also lets us block VS integration tests on this work before attempting to test state.

Without this, it's impossible to know when ProjectItem.CodeModel is going to return a result.

[AbhitejJohn]

Happy to test this out when we have one.

[AbhitejJohn]

@davkean : Did we get a chance to schedule this work in yet...

[davkean]

We're doing this as part of the language service hookup rewrite in 15.8. First part is in review, but we've only started on 15.8 this week. We're at least a sprint away from it.