cubicweb #2604664 cleanup the datafeed api [open]

as of today, the datafeed source uses a "parser" vreg-selectable object to do the bulk & the specifics of the import work

however the call is split in two phases:

.process, then
.handle_deletion

We want .process to handle the deletion itself.

prioritynormal
typeenhancement
done in<not specified>
load1.000
load left1.000
closed by<not specified>