* Added `AppliesHeadersInterface` to make applying headers to a request based
  on the body more generic and not specific to `PostBodyInterface`.
* Reduced the number of stack frames needed to send requests.
* Nested futures are now resolved in the client rather than the RequestFsm
* Finishing state transitions is now handled in the RequestFsm rather than the
  RingBridge.
* Added a guard in the Pool class to not use recursion for request retries.
