SCHEMA-886. Implement asynchronous writing using AsyncHBase.

Implemented and tested AsyncHBaseAsyncKijiBufferedWriter to
support asynchronous writing to a Kiji table.

issue: https://jira.kiji.org/browse/SCHEMA-886
I'm pretty sure this comment is copied from another implementation of this inner class, but it's a little misleading because we log whether the capsule is null or not, just a different message.