Bumping version to 2.1.0 and multiple bug fixes.

- Bumping version to 2.1.0
- Fix for workflow_client not using user supplied data converter
- Fixing a bug that was preventing CancellationExceptions initialized without a reason to go through
- Fixes to workflow/activity output/exception size bugs. Fixed and refactored truncation logic into a utility and wrote more unit and integ tests to confirm large workflow/activity responses/exceptions are being handled correctly.
- Fixed fork count bug in the runner