We need a way to configure the cql interpreter and runtime. So far we relied
on accessing the configuration class via various backdoors, but that causes
its own problems around initialization order and testability. To avoid that,
this patch adds an empty cql_config class and propagates it from main.cc
(and from tests) to the cql interpreter via the query_options class, which is
already passed everywhere.
Later patches will fill it with contents.