DCOM-311: Design API to convert values to DB-level values

Jira issue originally created by user @ocramius:

We currently use DBAL types to convert from PHP-side values to DB values:
1. we need to stop referencing types by string: the conversion should be part of the mapping
2. we need to be able to dump the code for the conversion (to allow inlining)
3. the DB value definition must own:
- the column where the value needs to be saved
- the table where the value is saved (eases STI/JTI decisions)
- the strategy to fetch the field (lazy/eager/etc)