Refactor: Add shared base classes for Cursor and Connection

[bgunebakan]

Part of crate/ecosystem#54.

Summary of the changes / Why this is an improvement

Splits Cursor and Connection into a shared base class and the sync class, the planned async client can reuse the parts that do no IO.

• CursorBase in crate/client/_cursor_base.py holds the cursor state
• ConnectionBase in crate/client/_connection_base.py holds the converter and time zone, the closed state
• Cursor and Connection keep all their public methods, signatures and docstrings.

This is a pure refactor with no behavior change, and no changelog entry because nothing changes for users.

Two internal details changed, they are not visible from public API.

• Cursor.fetchone(), fetchmany() and fetchall() no longer call self.next(), so a subclass that overrides next() no longer changes them.
• Connection._closed is now set before the server-version probe instead of after it.

Note to reviewer

The PR has 5 commits, and each one passes the tests on its own.

Checklist

• Link to issue this PR refers to (if applicable): Fixes https://github.com/crate/ecosystem/issues/59