* Implement Actor checkpointing
* docs
* fix
* fix
* fix
* move restore-from-checkpoint to HandleActorStateTransition
* Revert "move restore-from-checkpoint to HandleActorStateTransition"
This reverts commit 9aa4447c1e3e321f42a1d895d72f17098b72de12.
* resubmit waiting tasks when actor frontier restored
* add doc about num_actor_checkpoints_to_keep=1
* add num_actor_checkpoints_to_keep to Cython
* add checkpoint_expired api
* check if actor class is abstract
* change checkpoint_ids to long string
* implement java
* Refactor to delay actor creation publish until checkpoint is resumed
* debug, lint
* Erase from checkpoints to restore if task fails
* fix lint
* update comments
* avoid duplicated actor notification log
* fix unintended change
* add actor_id to checkpoint_expired
* small java updates
* make checkpoint info per actor
* lint
* Remove logging
* Remove old actor checkpointing Python code, move new checkpointing code to FunctionActionManager
* Replace old actor checkpointing tests
* Fix test and lint
* address comments
* consolidate kill_actor
* Remove __ray_checkpoint__
* fix non-ascii char
* Loosen test checks
* fix java
* fix sphinx-build
Hao Chen