refactor: standard database functions (#985)

- [x] factor out SQLite specific functions from base class
- [x] `HANAService` also requires `db-service/cql-functions` and then
also supports `floor`, `ceiling`, `average` …
- [x] kill `totalseconds`
- [ ] (later) `cql-functions` in the db-service becomes `sql-functions`
and a new interface `cql-functions` is added containing the set of
functions which shall be implemented by the dbs themselves this would
enable also overwriting/re-using `cql-functions` for non-sql dbs