Documenting SLA stats.

Bugs closed: AURORA-528

Reviewed at https://reviews.apache.org/r/22697/
Add test coverage for ResourceCounter.

Reviewed at https://reviews.apache.org/r/22942/
Database-backed implementation of QuotaStore.

Bugs closed: AURORA-421

Reviewed at https://reviews.apache.org/r/21459/
AURORA-231: Upgrade gradle license plugin to v9 which contains reduced debug output

Upgrades license plugin to latest version which contains updates to
silence ignored headers [1].

[1]: https://github.com/hierynomus/license-gradle-plugin/pull/31

Testing Done:
./gradlew -Pq clean build

Bugs closed: AURORA-231

Reviewed at https://reviews.apache.org/r/22956/
AURORA-552: License headers missing

Adds missing license headers

Bugs closed: AURORA-552

Reviewed at https://reviews.apache.org/r/22976/
Generate wrapper classes for all thrift structs in api.thrift.

Reviewed at https://reviews.apache.org/r/22998/
Fix build break in code generator.

Reviewed at https://reviews.apache.org/r/23087/
Fixing isort-check (no newline at the end) introduced by:
https://reviews.apache.org/r/22976/
Changing maintenance command to drain SLA-compliant hosts in a group.

Bugs closed: AURORA-542

Reviewed at https://reviews.apache.org/r/22842/
Use immutable types in AttributeStore.

Reviewed at https://reviews.apache.org/r/22999/
Revert "Returning a message when no tasks to kill."

Backing out of this change as there is no clean way to fix this now.

Bugs closed: AURORA-564

Reviewed at https://reviews.apache.org/r/23191/
Add a script that allows for easy selective rebuilding of components in the vagrant environment.

Reviewed at https://reviews.apache.org/r/22948/
Fix TaskObserver for json endpoint usage