Fixing @Timed method visibility.

Bugs closed: AURORA-966

Reviewed at https://reviews.apache.org/r/28883/
Use pip to pre-fetch python dependencies.

Reviewed at https://reviews.apache.org/r/28915/
Changing the default --batch-size to 1.

Bugs closed: AURORA-961

Reviewed at https://reviews.apache.org/r/28831/
Adding PMD rule to check @Timed annotation placement.

Bugs closed: AURORA-967

Reviewed at https://reviews.apache.org/r/28914/
Adding JMH framework support for scheduler performance analysis.

Reviewed at https://reviews.apache.org/r/28710/
More logging in MaintenanceController.

Bugs closed: AURORA-935

Reviewed at https://reviews.apache.org/r/28806/
Improving logging experience in admin drain_hosts.

Bugs closed: AURORA-943

Reviewed at https://reviews.apache.org/r/28811/
Suppressing redundant client command error messaging.

Bugs closed: AURORA-965

Reviewed at https://reviews.apache.org/r/28876/
Make abstract decorators effective in CommandHook class

Reviewed at https://reviews.apache.org/r/28879/