twfb: Add preliminary database layer.

This supports only what is used by the benchmark, but supports both
MySQL Client API and SQLite 3.
twfb: Wire up the database abstraction layer.

Things are not yet working, not even SQLite. MySQL hasn't even been
tested. But it is already building.
twfb: Fix SQLite portion of database layer.

db, queries, and fortunes benchmarks are running.
twfb: Make MySQL portion of database work.