SCHEMA-793. Implement AsyncKijiResult.

Implemented AsyncKijiResult, AsyncPagedKijiResult, AsyncMaterializedKijiResult,
AsyncKijiResultScanner, AsyncDataRequestAdapter, and associated tests.

issue: https://jira.kiji.org/browse/SCHEMA-793
Ugh.  Yah that's fine.  I haven't checked if you are/aren't doing this, but make sure you either can't overwrite that field with a new one, or if you do, make sure you close the old one.