Implement sqlite's writeCombine
Drop FIXME

execute is for emergencies, in particular when our sql does not fit the DB's sql.
update does not need Schema, drop it
Drop unused Cursor class
Resolve FIXMEs around DbConnection

- delete returns Int
- AnyRef are ok for insert
- don't structure insert, delete, update further (for now)
Drop unused import
Resolve some FIXMEs around relational's Expr
Expand LiteralNumber to LiteralInteger, ...Double, ...Decimal
QuerySelectFrom -> QuerySelectFromQuery
Drop FIXMEs

Methods are alredy put in util module