Wrap reading+writing workspace state operations with mutexes.

[jlrobins]

Summary of Changes

• The methods in ResourceManager which partially mutate their key values using a pattern like:

  1. Async read the existing key value as either an array or a map.
  2. Mutate that array or map.
  3. Async write the mutated array or map back to workspace storage.

  could potentially race with concurrent callers, and while the first caller was performing either steps 1 or 3, a concurrent caller number two could be scheduled which completes 3, meanwhile the first caller operated on stale data and its step 3. will revert what the second caller wrote.

• Fix using async mutexts (one per each workspace storage key) guarding across steps 1..3 above, serializing the concurrent callers. New utility method runWithMutex(key, callback) will run the given callback (the original body of the method performing 1..3) while holding the mutex corresponding to the workspace storage key being partially mutated.

• While it is unclear if a user could be fast enough to cause such a race condition, best to do things in a more concurrent-safe manner.

Any additional details or context that should be provided?

• Existing tests over ResourceManager identified one in-development issue (a missing await) during implementation, yay.
• Closes Guard resource manager sub-family methods with async-mutexes #534

Pull request checklist

Please check if your PR fulfills the following (if applicable):

Tests

• Added new
• Updated existing
• Deleted existing

Other

• All new disposables (event listeners, views, channels, etc.) collected as for eventual cleanup?

• Does anything in this PR need to be mentioned in the user-facing CHANGELOG or README?
• Have you validated this change locally by packaging and installing the extension .vsix file?

  gulp clicktest

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