Cancel long poll if a Future returned from getResultAsync is not referenced anymore by the user code

[Spikhalskiy]

Is your feature request related to a problem? Please describe.

If we hit WorkflowStub#getResultAsync and after that, we either don't wait on the result at all or wait for a bit and discard after, the long poll continues to run.
This creates a need for an unpleasant, unconventional WorkflowStub#getResultAsync(resultType, timeout).
Why do users need to pass a timeout to getResultAsync if they pass it into Future.get(timeout).
Most users just use WorkflowStub#getResultAsync(resultType) not understanding that they create a leak if the workflows don't finish fast.

Describe the solution you'd like
Instead of that, Temporal Java SDK code should maintain a weak or phantom reference on the CompletableFuture that we return to the users and cancel the background long poll when it's discarded by the user code.