Implement frame-work agnostic rollout and training workers (#137)

* Added checkpoint state file to coach checkpointing.

* Removed TF specific code from rollout_worker, training_worker, and s3_data_store

rl_coach/tests/test_checkpoint.py

@@ -0,0 +1,24 @@
+import os
+import pytest
+import tempfile
+
+from rl_coach import checkpoint
+
+
+@pytest.mark.unit_test
+def test_get_checkpoint_state():
+    files = ['4.test.ckpt.ext', '2.test.ckpt.ext', '3.test.ckpt.ext', '1.test.ckpt.ext', 'prefix.10.test.ckpt.ext']
+    with tempfile.TemporaryDirectory() as temp_dir:
+        [open(os.path.join(temp_dir, fn), 'a').close() for fn in files]
+        checkpoint_state = checkpoint.get_checkpoint_state(temp_dir, all_checkpoints=True)
+        assert checkpoint_state.model_checkpoint_path == os.path.join(temp_dir, '4.test.ckpt')
+        assert checkpoint_state.all_model_checkpoint_paths == \
+               [os.path.join(temp_dir, f[:-4]) for f in sorted(files[:-1])]
+
+        reader = checkpoint.CheckpointStateReader(temp_dir, checkpoint_state_optional=False)
+        assert reader.get_latest() is None
+        assert len(reader.get_all()) == 0
+
+        reader = checkpoint.CheckpointStateReader(temp_dir)
+        assert reader.get_latest().num == 4
+        assert [ckp.num for ckp in reader.get_all()] == [1, 2, 3, 4]