Move DB-specific SQL generation into DBAL

Currently there are many places throughout the code that switch on DBAL (more precisely, they want SQL dialect) and generate different SQL for different databases.

This should be accomplished by having methods on DBAL classes that generate SQL for generic constructs that is appropriate for the respective database.

Replacing all switch statements may be too ambitious of an undertaking, however new DB-specific logic should go into DBAL and existing logic may be converted where appropriate.