Still a bunch of open questions, but this is reasonably functional at least.

Necessary first step for safe shutdown.

Divorce selecting the appropriate broker with subscribing to that broker, they
have to be separate actions.

Thanks to Aaron Kavlie for finding the bug.

Also handle if the request ends up empty because all of the subscriptions were
clean.

Add a missing return, and zero out the subscriptions when we're done.

Cover POM init, setting and getting offsets.

Separate out test bootstrapping into init function.

Includes fix to error returns in offset_manager.go

Includes small fix to offset manager -- return to avoid
checking nil response.

- Make test function names more verbose (for targeting with -run)
- safeClose io.Closers to catch errors
- Remove debugging time.Sleep call
- Don't use globals for client, brokers, etc.

Add Close() OffsetManager interface

… done, so we can register a new one later.

Offset manager: make initial offset configurable