Refactor Internal Requests, Results to use Structures

Some good candidates for this:

• The tuples in connection.Connection._requests's values (linked above)

• The tuples returned by cluster.ResponseFuture.result (linked above)

• The tuples returned by pool.HostConnection.borrow_connection

We've taken a (very slightly) different approach in the DSE driver. Compare this C* driver code to its counterpart in the DSE driver codebase – the DSE driver sets values on a ResultMessage object that gets passed around.

Example patch for this, found during cleanup of some old branches:

The code changed a bit since the ticket creation. It is still valid for:

• connection.Connection._request

• pool.HostConnection.borrow_connection

I don't think this need to be done in a major.