Revisit ActivityExecutionContext and ActivityCompletion clients

[Spikhalskiy]

Code quality and readability around ActivityExecutionContext, ActivityCompletionClient, ManualActivityCompletionClient requires refactoring to make code more maintainable.

• [x] #943
• [x] Manual Completion Client is a public user-facing interface that is undocumented. #1181
• [ ] Errors handling around heartbeating in ActivityExecutionContext and ManualActivityCompletionClient are different without apparent reasons.
• [ ] #1233
• [ ] In case of some exceptions, ActivityExecutionContext stops heartbeating.
• [ ] ActivityExecutionContext#heartbeat and other methods like that should have a short explanation about the heartbeat throttling
• [ ] #1354

[Spikhalskiy]

ActivityExecutionContext is done