TaskAction: Add JsonSubType for LockTryAcquireAction
IndexTask: Remove unnecessary args to determinePartitions.
IndexTask: Call plumber.startJob()
OverlordResource: Fix comment
RemoteTaskRunner changes to make bootstrapping actually work.

- Workers are not added to zkWorkers until caches have been initialized.
- Worker status we haven't heard about will be added to runningTasks or
  completeTasks as appropriate. 
- TaskRunnerWorkItem now only needs a taskId, not the entire Task. This makes
  it possible to create them from TaskStatus objects, if that's all we have.
- Also remove some dead code.