Remove temporary MasterXaction objects

[yadij]

The ::Server hierarchy and Foo::NewServer() construction parameters taking MasterXactionPointer existed only as preparation for the vetoed PR #865 change.

With that implementation no longer happening this code now results in unnecessary memory allocation of MasterXaction objects which will be dropped immediately. Revert to original (circa Squid-3) construction parameters and remove TODOs which are no longer be relevant without the rejected design.