Implement listDatastores

Implement listDatastores API on DatastoreManager, it returns
a List<String> of all datastores for that instance.
Could be more self-describing?

```java
        for (File file : dsManagerDir.listFiles()) {
            boolean isStore = file.isDirectory() && Arrays.asList(file.list()).contains("db.sync");
            if (isStore) {
                //replace . with a slash, on disk / are replaced with dots
                datastores.add(file.getName().replace(".", "/"));
            }
        }
```