* Piggybacking the updating of user tag list.
When a new task is being added to a challenge, added code to update user tag list along with the existing code that already involves syncing / updating the user documents of challenge members.
* Update comment on number of simulatenaeous users to be updated concurrently in TaskQueue.
* Added comment to explain previous commit caaca469f8 (Update comment on number of simulateneous users to be updated concurrently in TaskQueue)
* Added unit tests for testing commit caaca469f8 (Update comment on number of simulateneous users to be updated concurrently in TaskQueue)
* Removed unused lines from newly added test cases
* Implemented lint suggestions
* Update code with changes requested in PR
* Implement atomic user challenges update
Prevents multiple concurrent requests from passing through
Fixes#11295
* Move user challenges update to separate method
* Rename challenge syncToUser to syncTasksToUser
Now that adding the challenge to user is separated, this methods main purpose is to sync the tasks
* Fix lint errors