config_file and db::config are soon not going to be copyable. The reason is that
in order to support live updating, we'll need per-shard copies of each value,
and per-shard tracking of references to values. While these can be copied, it
will be an asycnronous operation and thus cannot be done from a copy constructor.
So to prepare for these changes, replace all copies of db::config by references
and delete config_file's copy constructor.
Some existing references had to be made const in order to adapt the const-ness
of db::config now being propagated (rather than being terminated by a non-const
copy).
b146fd1356