* stage: make runs checkpointable
* api: add make_checkpoint API call
* tests: add test for checkpointed stage
* experiments: implement checkpoints callback chain
* initial working state
* use .dvc/tmp for checkpoint signal file location
* force repro for checkpoint experiments
* checkout: add flag to allow missing persistent outputs
(i.e. not yet created)
* use allow_persist_missing on checkpoint experiment repro
* executor: force checkout before checkpoints experiment repro
* move checkpoint commands to `dvc exp run`
* support resuming checkpoint runs with --continue
* fix git bugs
* experiments: include checkpoint commits in `exp show`
* use 1sec sleep timers
* fix apply_workspace test conflict
* tests: add test for `dvc exp run`
* cleanup signal file behavior
* fix bool conversion
* tests: add tests for checkpoint and checkpoint_continue
* fix styling
* fix checkpoint stage test
* fix checkpoint monitor thread condition/notification