client/network: Consistently track request arrival time

With https://github.com/libp2p/rust-libp2p/pull/1886/ one is guaranteed
to receive either a `ResponseSent` or a `InboundFailure` event for each
received inbound request via `RequestResponseEvent::Message`. Given this
guarantee there is no need to track arrival times in a best-effort
manner and thus there is no need to use a LRU cache for arrival times.