fix(util): preserve in-flight request state during unordered conversion

[Alpy16]

Description

This fixes a state-loss bug in CallAll::unordered.

CallAll can temporarily hold a request in curr_req after it has been pulled from the input stream but before it has been submitted to the inner service. Previously, converting a CallAll into CallAllUnordered rebuilt the unordered stream from only the service and stream, which discarded curr_req and reset other state.

Solution

• Preserve the existing CallAll state when converting to CallAllUnordered.
• Carry over curr_req and eof during the transition.
• Replace the ordered queue with a fresh FuturesUnordered queue.
• Add a regression test covering conversion while a request is in flight.

Fixes #860.