Fix race condition initialising ODCEnvironment objects

[SpacemanPaul]

Reason for this pull request

ODCEnvironment do not actually read from config at creation time. The actual reading of config has be deferred until after environment aliases have been processed, but this creates a race condition in multi-threaded environments.

Proposed changes

• Create an environment thread lock at creation time.

• Perform the deferred config reading within that environment lock.

  • [x] Tests added / passed
  • [x] Fully documented, including docs/about/whats_new.rst for all changes

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.43%. Comparing base (bcd71a2) to head (dc13fab). Report is 33 commits behind head on develop-1.9.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop-1.9 #1596 +/- ## ============================================ Coverage 85.43% 85.43% ============================================ Files 140 140 Lines 15493 15496 +3 ============================================ + Hits 13236 13239 +3 Misses 2257 2257 ```

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