les: fix retriever logic (#16776)

This PR fixes a retriever logic bug. When a peer had a soft timeout
and then a response arrived, it always assumed it was the same peer
even though it could have been a later requested one that did not time
out at all yet. In this case the logic went to an illegal state and
deadlocked, causing a goroutine leak.

Fixes #16243 and replaces #16359.
Thanks to @riceke for finding the bug in the logic.