command to trigger stream recovery

[jkarneges]

The HTTP streaming reliability feature makes a request to the origin server if received messages are incorrectly sequenced or after a period of inactivity. For quiet channels, this may mean that it takes awhile to recover. If the user is aware of publishing issues (e.g. the publisher crashed), it would be useful if the user could explicitly invoke the recovery request rather than having to wait for new messages to be published or for the timeout to expire.

Consider exposing a new command (via zmq command interface as well as HTTP) called recover, with potential parameters to fine tune:

• channel: recover only subscribers of a certain channel.
• channel-prefix: recover any subscribers that match.
• prev-id: recover only subscribers who previously received an item with this ID.

Default would be to recover all subscribers.

[jkarneges]

A basic version of this is implemented in the develop branch. It doesn't have any args and always recovers all subscribers.