Use Futures instead of latches to synchronize request processing

* Framework version: 2.0
* Implementations: All

## Scenario
The framework currently uses latches on the `HttpServletResponse` implementation to wait for the completion of a request. We should change this to use Futures. This need for this was highlighted in #272 as throwing an exception without releasing the latch may cause memory leaks.

Start by tracing the HttpServletResponse implementation and the latch lifecycle used during request processing, then review issue #272 for the exception path that can leave the latch unreleased. Done means request completion uses Futures instead of latches across all implementations without retaining resources when an exception occurs.

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