Database middleware abstraction

[Eric Zhao]

The database middleware can be the abstraction of the whole database operations.

This can be a big change for both the entire architecture and service components:

• The database middleware should support cache (e.g. Redis), simple load balancing and scalability (database sharding)
• The database middleware should be HA in production environment (so should be fault-tolerant
• The service components should operate the DB via the database middleware rather than directly connect to database

In addition, the internal DB library can be replaced with a more convenient and reactive one (maybe JOOQ).

The performance of this middleware component is of vital importance so this should be tested well.