Don't assign trace data to Statements

Depending on how a statement is executed (sync vs. async), sometimes trace data is transferred onto a Statement, sometimes not.

I don't think any request-specific results (trace information, warnings, custom payloads) should be attached to a statement. Instead, they should be kept associated with the ResponseFuture.

This task is to normalize handling of these data and update documentation.

It will require at least one update to cqlsh when trace_id is removed from Statement.