This field on the null shard is properly initialized
in maybe_init_schema_commitlog function, until then
we can't make decisions based on its value. This problem
can happen e.g. if add_column_family function is called
with readonly=false before maybe_init_schema_commitlog.
It will call commitlog_for to pass the commitlog to
mark_ready_for_writes and commitlog_for reads _uses_schema_commitlog.
In this commit we add protection against this case - we
trigger internal_error if _uses_schema_commitlog is read
before it is initialized.
maybe_init_schema_commitlog() was added to cql_test_env
to make boost tests work with the new invariant.
ce0ee32d5a