FORK: RPCs might block if fork channel or fork stack is not available

When we have nodes A,B,C,D, but fork-stack "fs-2" is not available on B, or fork-channel "ch-3" is not available on B, then an RPC invoked by A on all cluster nodes will time out.

Solution

• Throw an exception on B if a fork-stack or -channel is not available on a target node. This way, the RPC would return quickly and B's response would be set to the exception (e.g. "fork channel fc-2 not available").