Fix WorkflowQueue#offer and increase test coverage

[natemort]

What changed?

• WorkflowQueue#offer is misinterpreting the return value of WorkflowThread.await, resulting in it returning false whenever it can add an element to the queue or throwing an exception whenever it can't and the operation times out.
• Added additional test coverage for CompletablePromise

Why?

• Fix existing functionality

How did you test it?

• Unit tests.

Potential risks

Release notes

Documentation Changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.06%. Comparing base (c04bf6b) to head (852966c).
Report is 1 commits behind head on master.

Additional details and impacted files

Files with missing lines	Coverage Δ
.../uber/cadence/internal/sync/WorkflowQueueImpl.java	86.04% <100.00%> (+62.79%)	⬆️

... and 8 files with indirect coverage changes

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