Implement configurable external storage

[austin-hilberg]

Implement configurable external storage

[dimitri-yatsenko]

Required development

1. Separate blobs serialization from database query functionality datajoint/mym#10
2. Implement store specification in a configuration file. Maybe use dj.set as with other options or implement a json configuration file similar to datajoint-python.
3. Implement table declaration support for blob@store, including reverse engineering in .describe
4. Implement UUID checksum matching datajoint-python
5. Implement external store tracking table matching datajoint-python
6. Impelement the path subfolding scheme matching datajoint-python
7. Update insert to write blobs to the external file
8. Update fetch to read blobs from the external file
9. Impelement the delete and file cleanup methods matching those of datajoint-python

[shenshan]

Decisions:
2. Configuration

• potential retirement of dj.set,
• configure with dj_local_conf.json,
• inherit from mapping object?
• global and local config: order of default, global, local, env?

3. check the availability of uuid support from MATLAB
   https://www.mathworks.com/matlabcentral/answers/240447-using-java-uuid-in-matlab#answer_191523
   If too hard, we can skip the support of uuid.