Actor checkpointing with object lineage reconstruction (#1004)

* Worker reports error in previous task, actor task counter is incremented after task is successful

* Refactor actor task execution

- Return new task counter in GetTaskRequest
- Update worker state for actor tasks inside of the actor method
  executor

* Manually invoked checkpoint method

* Scheduling for actor checkpoint methods

* Fix python bugs in checkpointing

* Return task success from worker to local scheduler instead of actor counter

* Kill local schedulers halfway through actor execution instead of waiting for all tasks to execute once

* Remove redundant actor tasks during dispatch, reconstruct missing dependencies for actor tasks

* Make executor for temporary actor methods

* doc

* Set default argument for whether the previous task was a success

* Refactor actor method call

* Simplify checkpoint task submission

* lint

* fix philipp's comments

* Add missing line

* Make actor reconstruction tests run faster

* Unimportant whitespace.

* Unimportant whitespace.

* Update checkpoint method signature

* Documentation and handle exceptions during checkpoint save/resume

* Rename get_task message field to actor_checkpoint_failed

* Fix bug.

* Remove debugging check, redirect test output

src/common/task.h

@@ -135,6 +135,8 @@ UniqueID TaskSpec_actor_id(TaskSpec *spec);
  */
 int64_t TaskSpec_actor_counter(TaskSpec *spec);
 
+bool TaskSpec_actor_is_checkpoint_method(TaskSpec *spec);
+
 /**
  * Return the driver ID of the task.
  *