Coordinator drop/remove related stuff.

- DruidCoordinatorCleanup should wait for availableSegments to populate
  before dropping any segments.
- Clarify that millisToWaitBeforeDeleting applies to "removing" rather than
  "dropping" segments.
- LoadRule shouldn't need to wait for the deletionWaitTime before dropping
  excess replicas.
Merge pull request #661 from metamx/drop-remove-stuff

Coordinator drop/remove related stuff.