SQLite unlock notify is not handled for BEGIN statements

[madadam]

unlock notification was implemented in https://github.com/launchbadge/sqlx/pull/1658 but only when calling sqlite3_step. The BEGIN statement is executed using sqlite3_exec and the unlock notification is not handle there. This means that one can still get SQLITE_LOCKED_SHAREDCACHE errors when running multiple transactions concurrently.

The fix I think is to do something like this also here.

I'm happy to contribute a PR to fix this.

[abonander]

Please do!