Migrate to Futures in SigningQueue (#6689)

* oneshot channels instead of custom promises

* Future instead of handle_dispatch

* Even less copying

* Those explicit waits were a mistake, thanks, @tomusdrw

* No more unsafe polling

* Test for the new `is_done()` method

* Mark Futures as `#[must_use]`

* Solve most compilation warnings

* `try_ready!` is more ideomatic

* Turn spaces into tabs

* Documentation and visibility improvements

* Minor code style improvements

* Make Futures run on an explisit reactor

* Another round of code style issues

* Simplify ConfirmationReceiver type

* Flatten ConfirmationOutcome into a plain Result type

* Get rid of a separate `pending` set, it was a stupid idea

* Clarify `add_request` docs

* No need to reduce the scope of the mutex here