Replaced the old `read_barrier` helper from "test/pylib/util.py"
by the new helper from "test/pylib/rest_client.py" that is calling
the newly introduced direct REST API.
Replaced in all relevant tests and decommissioned the old helper.
Introduced a new helper `get_host_api_address` to retrieve the host API
address - which in come cases can be different from the host address
(e.g. if the RPC address is changed).
Fixes: scylladb/scylladb#19662Closesscylladb/scylladb#19739
9ab25e5cbf