Define block open methods for Connection and BrokerPool

@bpot Follow-up to #54, adds block methods for resources to help make sure things get cleaned up.

Somewhat related, I noticed half of the resource/connection-like things in this gem have `#shutdown` and the other half have `#close` so I decided to give it a shot making them all `#close` (with aliases to `#shutdown` for clients of this gem). Let me know what you think!