Release 3.0 checklist

[JanJakes]

This is a checklist of issues to complete before tagging version 3.0:

• Restrict SQLite installation permissions #464
• Refine the 3.0 API surface #469
• Review PDO API coverage (WP_MySQL_On_SQLite and WP_MySQL_On_SQLite_Statement).
• Review WP_MySQL_On_SQLite constructor $options and their compatibility with PDO.
• Unify the emulated MySQL version.
• Update README and agent files, remove references to the legacy driver.
• Prepare changelog.

Likely post-3.0:

• Snapshot column metadata into PDO statement rather than storing them statefully in WP_MySQL_On_SQLite. Implement PDOStatement::getColumnMeta(), deprecate get_last_*() methods.
• SQLite driver exception doesn't populate $errorInfo.
• Create composer packages.
• Multisite installation support.