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.

Written by the indexing model from the issue text.