Xfrout3b: extract time-consuming ops from XfroutSession

This is another round of refactoring of xfrout.

The main change is to extract the part of XfroutSession that builds and sends xfr response into a separate new class, XfroutResponder.  The intent is to eventually make the responder class is the only one run by a separate thread.  That would allow us to use generic data source configuration and (mostly) get rid of sqlite3 specific code.

Some method names were modified, but this is essentially a straightforward refactoring.