[db] Wrap an edn::symbols::NamespacedKeyword rather than a String in db::TypedValue

[ncalexan]

Per #171 (comment). Right now, we extract and clone a String in db::TypedValue::Keyword. We'd like to keep this kind of data structured as much as possible, but I don't want to do this work for #170 before landing, so I'm tracking it here.

[ncalexan]

I think this should be pretty straight-forward.

[jsantell]

Blocking on #291, much of the same code