* 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.