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
• Expand PDO API coverage #471
• Document the 3.0 driver API #477
• Use synchronous=NORMAL or synchronous=FULL?
• Review SQLite-specific PDO constructor option naming.
• Unify the emulated MySQL version.
• Update README and agent files, remove references to the legacy driver.
• Prepare changelog.

Likely post-3.0:

• Create composer packages.
• Multisite installation support.