History activity cancellation fixes

[mastermanu]

• Reduces Default Maximum History Size Error and Warning thresholds by a factor of 10. As we do further profiling / stabilization of Temporal, these are open to being increased in the future.
• Fixes nil-reference panic that would occur when we attempt to process a request to cancel an activity that has already been completed.
• Validated with Unit and Integration tests
• Minimal risk for the nil-reference panic fix.
• Moderate risk for the History Size reduction if an environment has workflows which already exceed this limit, in which case, the user can just override the settings via dynamic config before deploying