Refactor data type maps and fine tune column metadata

[JanJakes]

As a follow-up on https://github.com/WordPress/sqlite-database-integration/pull/209, there are a few small improvements to be done.

1. Currently, we already have multiple maps in the form of mysql-type-to-something. We can consolidate them into something like this:
2. The column metadata information can be made a bit more accurate. Namely—adding support for MySQLi flags and possibly improving some edge cases.