Add Close() OffsetManager interface

[wvanbergen]

This way, an OffsetManager that uses an external data store can properly clean up the resources it uses.

• Should we add AsyncClose() for consistency?
• I added a dummy implementation for the Kafka offset managers. Should be do something there now the function is present?

@eapache

[eapache]

Should we add AsyncClose() for consistency?

Does this make sense as a useful function for the ZK-based manager?

[eapache]

I added a dummy implementation for the Kafka offset managers. Should be do something there now the function is present?

Nope. The consumer is in a similar situation - all it does is (potentially) close the underlying client, but we don't even have that to deal with because the only constructor we provide is OffsetManagerFromClient.

[eapache]

Add AsyncClose if you think it's needed, then LGTM.

[wvanbergen]

I don't think we'll need AsyncClose, because there are no channels to wait for.