[devtools] Avoid db_cache collisions and mis-loading
This is done by stopping to share the db_cache between
TestDataBaseHandler instances
It may seem overkill, but is the only way I could get it to work
properly. I think the whole db caching code needs some rework to better
work with different db handlers.
Closes #4601328
usr/lib/PY_VERSION/*-packages/cubicweb/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/