New default/preferred dbm backend

[stoneleaf (Ethan Furman)]

@erlendaasland

(My PR does not make dbm.sqlite3 the new default; there is plenty of time to make it so before the feature freeze, if we want it that way.)

Shouldn’t we make it the default now so it can be tested in the alphas? We can undo the default portion if it turns out to be a problem.