MEDIUM: mux-quic: enforce thread-safety of backend idle conns

MEDIUM: mux-quic: enforce thread-safety of backend idle conns

Complete QUIC MUX for backend side. Ensure access to idle connections
are performed in a thread-safe way. Even if takeover is not yet
implemented for this protocol, it is at least necessary to ensure that
there won't be any issue with idle connections purging mechanism.

This change will also be necessary to ensure that QUIC servers can
safely be removed via CLI "del server". This is not yet sufficient as
currently server deletion still relies on takeover for idle connections
removal. However, this will be adjusted in a future patch to instead use
idle connections standard purging mechanism.