reserve

Takes one of the streams to address a forward is sent on, and returns it once taken. A transport that carries a bounded number of requests to a peer at a time waits here, in order, while they are all taken, with no bound of its own: the caller bounds the wait, and a cancelled call takes nothing. It does not fail: a peer that cannot be reached fails the ClusterStream.forward. The default takes nothing, and its stream forwards through forward.