When table exists in database and no jdbc-type defined, check column type for jdbc-type rather than just using default

[andyjefferson]

We currently just set the jdbc-type to the default for the java type. But if the datastore type is inconsistent with that then this leads to errors. Better option is to come up with a default based on the datastore column (when it exists). With JDO we don't want to allow this since there is a default defined there, so have it controlled via a boolean persistence property