Database schema review

[cannium]

@cxt90730 @dahefanteng @thesues

I think DB schema needs some review, for the following reasons:

• some columns were designed for HBase, and not fit TiDB/MySQL very well
• we need to support versioning (business pressure)
• some tables do not have an explicit primary key (WTF!?)
• intensively use null value (ref)
• intensively use JSON type (ref)
• use varchar for enumeration types
• mysterious names, e.g. multipartpart, multiparts, objectpart

[cannium]

Proposed schema, not finished yet:
https://gist.github.com/cannium/fbf2336ca1b99b8d1eb3c91346c1d198
Please leave in-line comments there.