JDBC connectivity from Airpal

One way for providing JDBC connectivity from Airpal is to move its lowest layer of DB connectivity (executeWith invocations of com.airbnb.airpal.core.execution.QueryCliemt: there is 1 for data and about 6 for metadata) to JDBC query execution. The JDBC results (mostly data and schema) can then be converted to presto client api equivalent objects and rest of the logic in airpal would follow.

Another approach is to rewrite airpal with native JDBC support by moving over to JDBC objects for internal use and communication as well. It looks like a much bigger change.

I am planning to add support for dynamically choosing between presto client or JDBC connectivity. I will use the com.airbnb.airpal.presto.QueryRunner to hold either a presto client session or a JDBC connection accordingly. I guess at that time it might be better to move QueryRunner class to com.airbnb.airpal.core.execution.