Support for importable types completion in C# completer

This adds support for completing types which are referable by full name, but not yet imported with a using directive. For example, this change will allow completion of "Console" even without a System namespace import. These imported type completions will include an indicator of the required namespace import, and will be sorted after all non-imported types.

Note that this change pulls OmnisharpServer forward, including many other changes. I have not personally encountered any issues using the OmnisharpServer head for past few weeks.