Make RazorProjectService operations atomic

The RazorProjectService APIs (e.g. AddDocumentAsync, UpdateProjectAsync, etc.) were chunked into multiple tasks and awaits. That means that other updates to the ProjectSnapshotManager could be interleaved with them. This change ensures that the operations are atomic (as far as the ProjectSnapshotManager is concerned) by running each within a call to ProjectSnapshotManager.UpdateAsync(...). Because of other changes to make CPU-bound async work synchronous the operations themselves can be made synchronous.

Author: DustinCampbell