Moving kill wait to the client (Part 2: server changes)

Bugs closed: AURORA-370

Reviewed at https://reviews.apache.org/r/20959/
Adding min_instance_count threshold to improve safe domain results.

Bugs closed: AURORA-305

Reviewed at https://reviews.apache.org/r/20986/
Fix broken test build.

Bugs closed: aurora-397

Reviewed at https://reviews.apache.org/r/21188/