Infra: Improve db retry handling

Use theh new poldb_setup hook to (a) set up HandleError and (b) run
the DGIT_RPD_TEST_DBLOOP_HOOK.

This is needed because the PRAGMA in poldb_setup can cause a
SQLITE_BUSY error in newer sqlite3, and we want both the HandleError
setup and the hook (which sets a timeout, amongst other things) to
occur before sqlite3 tries to take its lock.