Fix journal_mode = WAL race condition

[rkistner]

Checking journal_mode in tests could reproduce the issue quite consistently - journal_mode would be the default delete instead of wal as it should be.

The issue appeared to be related to opening the write and read connections for the new database at the same time. The error is never actually reported (return code is ignored), but I've seen similar issues with Flutter where it would return a DATABASE_LOCKED or similar error in the initial PRAGMA statements.

The workaround here is to run the initial PRAGMA statements in the main thread, right after opening the database. This ensures the write connection is opened first, initializing the database, before the read connections are opened.

This also sets PRAGMA busy_timeout which did appear to help when executing concurrently, but did not cover all cases.

This also fixes some potential lock issues around workQueue being checked without locking first. I'm not aware of any actual issues this caused, but in theory this should be more stable.