Actualize  features related with push-queues

- added required `name` field to subscribers
- actualize adding, replacing and removing of subscribers

Also:

- closed all unclosed and unused reader objects
This conflicts with this pull request: https://github.com/iron-io/iron_mq_java/pull/53, and I just realized that pull should have gone into v3. Can you copy what we did in that pull into this so it's the same/similar. I noticed you added a couple extra things here checking for null, etc, so keep those and add the other stuff.