Implement Cayenne-free lightweight JdbcConnector

[andrus]

In 99% of cases we don't really need Cayenne to read source data for LM jobs. A pure JDBC should suffice. It will be much more lightweight and can implement optimizations like #142 out of the box.

The only useful thing Cayenne does is velocity templates for SQL, that we can replace with simple parameter binding for PreparedStatement.

Also DataFrame has Cayenne-free JDBC connector already, which we can wrap in an LM connector.

I guess the current JdbcConnector can be renamed to CayenneConnector, and the name reused for this one.