TimeLockingFuture implementation is incorrect and easy to break

Expected Behavior

CompletableFuture returned from WorkflowStub#getResult and WorkflowStub#getResultAsync that has time skipping functionality built-in can correctly work with all CompletableFuture methods without losing time skipping functionality.

Actual Behavior

CompletableFuture that is returned from #getResultAsync when we use TestWorkflowService is TimeLockingFuture. It overrides CompletableFuture#get and CompletableFuture#join to provide time skipping functionality.
This implementation is extremely brittle because any chaining of this CompletableFuture will not call the overridden #get and #join. So even trivial chaining completely and unexpectedly breaks the time-skipping feature.

Steps to Reproduce the Problem

1. Use TestWorkflowService
2. Perform any trivial chaining on the result returning from getResultAsync
3. Time skipping doesn't work on #get call on a chained CompletableFuture from step 2.

Demonstration of the problem: https://github.com/Spikhalskiy/java-sdk/commit/e73d574428a7001534601c3bebabb60a23612bff
Adding a trivial result = result.thenApply(s -> s); chaining breaks the test and it hangs.

Proposed solution

The current solution with intercepting stub methods and extending CompletableFuture looks like a hack and I'm not sure if it's possible to make it work correctly with CompletableFuture implementation.
My proposal is to completely remove built-in time skipping from WorkflowStub and TimeLockingFuture.
One of the correct and nice ways to give time-skipping to tests developers can be through exposing of method .skipTime on some helper object that would return an AutoCloseable object that will trigger disabling of time-skipping at the end. It could be used by the test writers using the following pattern:

try (TimeSkipUtils.forStub(worflowStub).skipTimeSegment()) {
    result = resultFuture.get()
}

TimeSkipUtils.forStub can dig into worflowStub and make a helper that is aware of specific TestWorkflowService and provides 
.skipTimeSegment() that returns our ```AutoCloseable``` resource described above.

Specifications

• Version: Current master (5f2ac6133c6560984eff35cc85faafb85cbf2c94)

Start with WorkflowStub#getResultAsync, TimeLockingFuture, and TestWorkflowService, then reproduce the reported failure by applying thenApply to the returned CompletableFuture. Review how CompletableFuture chaining bypasses the overridden get and join methods. Done means the proposed time-skipping behavior remains reliable for chained futures, or the replacement test-facing API is defined and verified.