Keep track of the pending response for each peer individually (#13941)

* Keep track of the pending response for each peer individually

When peer disconnects or the syncing is restarted, remove the pending
response so syncing won't start sending duplicate requests/receive stale
responses from disconnected peers.

Before this commit pending responses where stored in `FuturesUnordered`
which made it hard to keep track of pending responses for each individual
peer.

* Update client/network/sync/src/lib.rs

Co-authored-by: Bastian Köcher <[email protected]>

* ".git/.scripts/commands/fmt/fmt.sh"

* Apply suggestions from code review

Co-authored-by: Dmitry Markin <[email protected]>
Co-authored-by: Sebastian Kunert <[email protected]>

* Update client/network/sync/src/lib.rs

---------

Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: command-bot <>
Co-authored-by: Dmitry Markin <[email protected]>
Co-authored-by: Sebastian Kunert <[email protected]>