Separate DBAL and driver interfaces

[morozov]

Please refer to the conversation in #3582 (comment). More details to be added.

While currently the wrapper and driver connections and statements implement the same connection and statement interfaces, they don't implement them in the same way and completely. At the same time, the interfaces represent a superset of the contract of the wrapper and driver classes.

1. The wrapper Connection class has two pairs of methods: executeQuery() + executeUpdate() and query() + exec() where the pairs are almost identical to the second. The first pair is mostly used at the DBAL level but the second is needed to implement the driver-level interface.
2. Driver statements have to support all fetch modes exposed via the abstraction (e.g. mixed or class) and therefore implement them instead of having the abstraction implementing the fetch modes.

[morozov]

Closed via #4159.

[github-actions]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.