As initially planned, if no trouble was reported on dynamic backend
commands on the CLI, the experimental status could be dropped before the
release. The feedback was not very broad, but was conclusive in that the
operations work as expected and the current syntax can be preserved even
for future evolutions. So we can drop the experimental status.
Complete delete backend regtests by checking deletion of a proxy with a
reference on an unnamed defaults instance. This operation is sensible as
the defaults refcount is decremented, and when the last backend is
removed, the defaults is also freed.
Add a reg-tests to test "del backend" CLI command. First, checks are
performed to ensure a backend cannot be deleted if not in the expected
state.
Then, a "del backend" success is tested. Stats are dumped to ensure the
backend instance is indeed removed.
