Use java.util.Objects replacements for guava helpers.

Reviewed at https://reviews.apache.org/r/22982/
Export zero for racks that have no lost tasks.

Bugs closed: AURORA-548

Reviewed at https://reviews.apache.org/r/23189/
Add configuration plugins to default aurora client.

- Add a plugin to allow users to set logging levels.
- Add a plugin to allow users to configure whether or not unknown exceptions get
  caught or revealed.

Bugs closed: aurora-544, aurora-545

Reviewed at https://reviews.apache.org/r/22839/
Log loaded config file at level TRANSCRIPT (aka INFO+1)

Have client commands that load config files print the contents of the loaded config file to log at level TRANSCRIPT (aka INFO+1).

Bugs closed: aurora-567

Reviewed at https://reviews.apache.org/r/23199/