Loading checkpoint does not work

The add_uid variable is not passed to the agent and learner, and is set to default to true in the Snapshotter and Checkpointer.

Therefore when passing to the agent the checkpoint path say:
```python
checkpoint_subpath = "/log/fed68b00-b8cb-11eb-812c-bba6f8f69ddd"
```
a new folder will be created inside it with a new identifier and the previous checkpoints are not loaded.

To reproduce run test_dqn.py twice, the second time set checkpoint_subpath pointing to the previous log path and pass it to the agent