[massive store] Lazy removal of constraints and metadata indexes
They should not be removed during store's init, because we may want to query the
database with its index between store creation and call to prepare_insert* (e.g.
to build the extid2eid map).
Along the way:
* rename drop_metadata_constraints into drop_metadata_indexes, because that's
what it does
* rework a bit impacted tests
Closes #15538359
usr/lib/python2*/*-packages/cubicweb/devtools/
usr/lib/python2*/*-packages/cubicweb/skeleton/
usr/lib/python2*/*-packages/cubicweb/test
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usr/lib/python2*/*-packages/cubicweb/entities/test
usr/lib/python2*/*-packages/cubicweb/ext/test
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usr/lib/python2*/*-packages/cubicweb/sobjects/test
usr/lib/python2*/*-packages/cubicweb/hooks/test
usr/lib/python2*/*-packages/cubicweb/web/test
usr/lib/python2*/*-packages/cubicweb/etwist/test