beam/inmem: Receiver and Sender don't need to implement Close
beam/inmem: fix the tests
beam/inmem: gofmt
beam/inmem.StackSender: a dynamic beam forwarded with health checks

StackSender forwards beam messages to a dynamic list of backend receivers.
New backends are stacked on top. When a message is sent, each backend is
tried until one succeeds. Any failing backends encountered along the way
are removed from the queue.
beam/inmem: NopReceiver and NopSender
beam/inmem: Sender implements Close
ur: Hub passes messages through a chain of dynamicly registered handlers