use zmq.PROBE_ROUTER to register connections to schedulers

There is a race where engine connections to schedulers takes a finite amount of time, and it is possible for clients to be informed of an engine's registration *before* the engine is fully connected to all schedulers.

The current registration design effectively assumes heartbeats will complete after registration is complete, but the faster launch-then-use nature of the cluster Python API reveals the race.

Using the zmq.ROUTER_PROBE should allow us to delay registration notification until all probe messages have been received by the schedulers.