Deduplicate getResultAsync calls inside WorkflowClient or WorkflowStub

[Spikhalskiy]

Is your feature request related to a problem? Please describe.
If users call getResultAsync on the same stub several times or if they call getResultAsync on different stubs for the same workflow execution, SDK will open a new long poll for each such request.

Describe the solution you'd like
SDK should deduplicate such calls and open only one long poll under the hood as an optimization, so users don't have to think about it in their code.

Additional context
This task should likely be paired with #1212, as both require changes in the internal structure of the same area of SDK code.