Registrar interface for registering channels.
Add a channel expiration time.
Implement redis registrar.
Use registrar when creating a stream.
Return 410 for unregistered streams.
Error from broker.Subscribe instead of using Registrar directly.
* Close channel early if it's already closed.
* Reopen a channel if it's been marked as done.
Merge branch 'unknown-stream'

Conflicts:
	redis.go