Indexing service stuff

*Don't merge yet! Tests don't pass!*

The TaskQueue directly manages the TaskRunner. The main management loop runs
periodically and checks that the runner is doing reasonable things. If not, it
attempts to adjust the runner. The management loop also runs on-demand when a
task is added to keep task assignment relatively low latency. The TaskConsumer
is no longer necessary and so it no longer exists.

Task interval locks are handled differently. Instead of some tasks acquiring
locks at runtime and some tasks having implicit fixed lock intervals, all tasks
ask for locks explicitly. This occurs either in "isReady" (which runs on the
overlord) or in "run" (which runs on the peon).

Other changes:
- The TaskQueue is attached to the leader lifecycle, instead of global
- The TaskLockbox is able to sync itself from storage and is no longer
  bootstrapped by the TaskQueue.
- RemoteTaskRunner does not clean up zk paths until asked to. This will
  prevent deletion of statuses that have not yet been committed.
- Added retries on DbTaskStorage operations.
- Removed SpawnTasksAction (no more subtasks)
- Removed obsolete EventReceiverFirehose configs
- Removed obsolete OldOverlordResource
- Removed TaskStorageQueryAdapter methods related to subtasks