Remove code that causes memory leaks (#888)
### Changes
- Replace two usages of `ConcurrentBag` with `ConcurrentStack`, which
has a much simpler implementation that has no risk of memory leaks.
ConcurrentBag was causing leaks. More information is here:
https://stackoverflow.com/questions/5353164/possible-memoryleak-in-concurrentbag
- Remove cancellation support from AsyncQueue, since the cancellation
token would gain a reference to the item being dequeued, which a caller
might not expect and which causes a memory leak if the cancellation
token is kept alive.
- Add a 'Clear' method to 'AsyncQueue' to allow cancelling all customers
- Fix a surprising memory leak in `CustomStackSizePoolTaskScheduler`
- Enable using an ExecutionEngine with a custom TaskScheduler
### Testing
- Tested with a profiler that the above changes resolve the issue that
each Boogie program created by Dafny was never garbage collected.
