circuit v2: relay service not cleaned up properly on close

[sukunrt]

There is a race condition between

relay.Close()

and

relay.handleReserve()

if relay.Close() is called around handleReserve execution at:
https://github.com/libp2p/go-libp2p/blob/master/p2p/protocol/circuitv2/relay/relay.go#L181
if relay.Close acquires the lock it'll untag the peers and cancel the background goroutine running the gc.
Then handleReserve will add the reservation and tag the peer with conn manager.