Quick fix at_lookup race condition

[gkc]

- What I did

• Added use of a mutex into AtLookupImpl._process() to prevent race condition which surfaced during testing the mwc_demo end-to-end

- How I did it

• Added dependency on mutex package
• Created a Mutex for use by _process(); modified _process() to acquire the mutex to start with, and release it in a finally block

- How to verify it

• DONE: Verified this fixed the issue for mwc_demo
• DONE: Run all tests in at_client_sdk with a dependency override for at_lookup to this branch
• DONE: test atmospherepro with this branch
• DONE: Added a test to at_client_sdk/at_functional_test. (To see the test failing, add at_lookup: 3.0.7 to at_client_sdk/at_functional_test/pubspec.yaml)

- Description for the changelog

• Added use of a mutex into AtLookupImpl._process() to prevent race condition

[gkc]

@murali-shris Please review at your earliest convenience; given the severity of this bug, I'd like to merge to trunk asap - ideally tomorrow

I will add extra tests tomorrow morning (unit tests for at_lookup, as well as an end-to-end test)

In the interim, @purnimavenkatasubbu it would be great if you could locally run all of the tests in at_client_sdk, and maybe test AtmosherePro as well, in order to verify that this doesn't break something else. You will need

dependency_overrides:
  at_lookup:
    git:
      url: https://github.com/atsign-foundation/at_libraries.git
      path: at_lookup
      ref: quickFixAtLookupRaceCondition

[gkc]

Issue documented at #140

[purnimavenkatasubbu]

Verified functional_tests of client_sdk locally and verified @mosphere_pro app pointing to the following in dependency_overrides
at_lookup:
git:
url: https://github.com/atsign-foundation/at_libraries.git
path: at_lookup
ref: quickFixAtLookupRaceCondition

Did not face any issues. Attaching POT's
atLookupTestLogs.txt

@mosphere_pro app recordings

sending_notifications.mp4

receiving_notifications.mp4

auth.mp4

[gkc]

Thanks Purnima, great!