Rework custom config requests

[Colengms]

There is a corresponding PR on the native side.

This PR removes the delivery of a custom configuration immediately before processing the didOpen, in favor of the newer mechanism of requesting each configuration as needed. The original approach predated needing configurations for other purposes, such as Find All References.

Benefits are:

• No need to look up and deliver configurations for files opened internally by VS Code and never active in the editor.
• Code simplification/removal. Removed QueryTranslationUnitSource request.
• Removing the potential for a config provider to stall our message queue due and delay delivery of didOpen.