Regular columns may have names of arbitrary type. See
https://issues.apache.org/jira/browse/CASSANDRA-8178
Primary key columns are UTF8.
This change also does some refactoring of the schema object to make
the change easier to digest (more encapsulation).
The method may defer so the result is wrapped in future<>.
I think we don't need to wrap arguments in shared_ptr<> because they
may come from the request state object.
