Feature/soledad adbapi3

DO NOT MERGE YET
just for testing / pullreq purposes

This was implemented when we were thinking about making it feasible to synchronize with many databases (URLs). Because of that, we were caching one synchronizer for each URL. This doesn't make much sense in the context of soledad, were there's one centralized database, whose URL is passed to the constructor upon initialization. Maybe we want to rethink the complexity of having multiple syncers cached here.