[dataimport] test for a value is in a set and insertion in a set should live together
also inline some methods on the way, in the hope to make the public api clearer
at some point, and only put valuable values in those sets so we don't have to
endlessly test for consistency.
[dataimport] use a single _initialized set instead of _entities / _rtypes
and rework a bit methods using them so that 'in' test and set insertion leave in
the same method.
[dataimport] drop massive store etype to eid index
and fix potential problem with is_instance_of relation insertion in case where
the class of inserted entity has some (yams) parent.