acc: Use granular locking in testserver

[denik]

Changes

• Make testserver.FakeWorkspace thread-safe.
• Make testserver.Server have granular locking for selecting/creating fake workspace. Otherwise it's unlocked.
• Make acceptance test runner use its own lock for recording requests. This can be reused with proxy implementation (Add support for recording requests in integration acceptance tests #2720).

Why

This makes use of testserver.Server as safe as it was before but makes handlers run concurrently. Note, although most handlers run quickly, there is Delay option that sleeps in the handler when lock is acquired. With this change, sleep is done when lock is released.

This enables us to refactor acceptance test server to reuse single server rather than create a new listening socket & server per test, which adds more and more overhead as we add more tests.

This also enables more advanced usage of testserver.Server where handlers do more I/O.

Note, there is no consistent performance win or lose with the current test suite.

Tests

Ran test suite many times with hyperfine to ensure it's running stable.

[shreyas-goenka]

Do we need this for the local server? Does this speed up test runs?

All handlers that do not have a delay configured should return very fast. There seems to only be one test configured with a delay (telemetry/timeout)

[denik]

Do we need this for the local server? Does this speed up test runs?

I expanded justification. This enables a few things but does not give a speed up on the current test suite.