DemoStorage:  Added implementations for registerDB() and new_oid().  As
Tres discovered the hard way, wrapping a ZEO client storage as a
DemoStorage base storage yields insane behavior otherwise.

BaseStorage.new_oid():  Rewrite to eliminate recursion, and hence also the
need for the undocumented and irregular `last=` argument.

Other:  removed the `last=` argument to new_oid() every place that felt
compelled to spread that insanity ;-).  Seriously, it served no purpose
at all elsewhere, and looks like people just cut 'n pasted in fear.