fix folly::FunctionScheduler.cancelFunctionAndWait() hanging issue

fix folly::FunctionScheduler.cancelFunctionAndWait() hanging issue

Summary:
When
- only one function is scheduled in FunctionScheduler; and
- the function is running while cancelFunctionAndWait() is being called;
FunctionScheduler.cancelFunctionAndWait() will hang forever.  The root cause is that the condition in cancelFunctionAndWait() is incorrect:

```
runningCondvar_.wait(l, [currentFunction, this]() {
  return currentFunction != currentFunction_;
});
```

because currentFunction will not be changed if only one function is in the scheduler.

The fix here is to
- clear currentFunction as nullptr.  This also makes the internal behaviors of cancelFunction() and cancelFunctionAndWait() consistent.
- introduces additional variable to indicate the state of cancelling current function.  After running the function, the background thread will detect cancellation of current function and clear the variable.
- cancelFunctionAndWait() condition variable will wait for the variable to be cleared.

Similarly, cancelAllFunctionsAndWait() also suffers from the same issue.

Unit tests are added to reproduce the issue.

Reviewed By: yfeldblum

Differential Revision: D5271664

fbshipit-source-id: acb223304d3eab23129907ce9ff5e57e55f1e909