Deprecate 'Sub' in favor of more general 'Ctl'

[jsccast]

A suggestion from @robertfarnum:

Introduce a Ctl method for Chan. Different channel types (e.g., MQTT, HTTPServer) can then support different Ctl requests. For example, the MQTT channel type should support subscribe and unsubscribe.

Currently, for several channel types, the method Sub will return a "not implemented" error. In other words, Sub doesn't make sense for many channel types. Also the corresponding Unsub (for "unsubscribe"), which does make sense for an MQTT client is not actually available today. In this proposal, the MQTT channel would support a "subscribe" Ctl request and an "unsubscribe" Ctl request. Possible example of the former:

- ctl:
    subscribe:
      topic: 'foo/+'
      qos 1

(Might want an explicit payload?)

How would a test get the results of that call? Perhaps via the usual Recv mechanism on a designed topic (e.g., ctl)?

- recv:
    topic: ctl
    pattern:
      success: true

?