[Bugfix] adding completableFuture docs and minor bug fixes

[zhongkechen]

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Issue Link, if available

Fix #97

Follow-up actions on previous PR #103

Description

• Added more comments to CompletableFuture related code
• Added a new design doc ADR-003 for CompletableFuture based operation coordination
• Removed duplicate method isTerminalStatus
• Fixed a bug in ChildContextOperation that doesn't handle SuspendExecutionException properly, causing flaky test cases
• Fixed minor issues with runners
  • Added handling of errors in CloudDurableTestRunner
  • Fixed LocalDurableTestRunner gets stuck when skipTimer is true
• Replaced some magic values in test cases with constants
• simplified thread local and active thread management
  • removed unset/reset thread local object from waitForOperationCompletion. Thread local object is set when the thread is started and it is recycled when the thread is terminated.
  • removed thread type from active thread tracker. Only thread id is needed to track active threads.
  • renamed some methods to better reflect the purpose and avoid confusion. Context is renamed to ThreadContext to reflect that it's stored in thread local.

Demo/Screenshots

Checklist

• I have filled out every section of the PR template
• I have thoroughly tested this change

Testing

Unit Tests

Have unit tests been written for these changes? Yes

Integration Tests

Have integration tests been written for these changes? Yes

Examples

Has a new example been added for the change? (if applicable) Yes