Do this by ways of directly using FileStorage and temporarily extending
it with load function which returns both serial and next_serial on load.

We currently need next_serial to be there in AnswerObject for neo/py
client cache - as its code currently depends on next_serial (= next_tid)
for its computations.

This however can be reworked and next_serial not used there at all.
See also 5886aad3 (X zodb: Clarify Load interface)
.