New API to wait for handler executions to complete and warnings on unfinished handler executions

[dandavison]

Closes #538

Implementation ready for review; names are still under discussion.

What was changed

• Added a new API to wait for update and signal handler executions to complete. Provisionally named workflow.all_handlers_finished().
• Issue warnings on workflow exit if any handlers are unfinished.
• Allow warnings to be disabled on a per-handler basis via a signal/update decorator parameter.

Why?

Workflows exiting while handler executions are in progress is an important category of run-time workflow incorrectness.

[dandavison]

This PR was missing one thing -- it didn't address cancellation. Discussed with @cretz in a different channel and we agreed that it was a bug that the CancelWorkflowExecution command was not being defined to be a workflow "completion" by the SDK. Recent commits fix that bug and add the test coverage that this PR had been missing regarding cancellation.