CASSGO-29 refactor packages to separate frontend api to backend internal api

Currently everything exists within the gocql package, as the driver functionality expands the package gets more and more complicated.

We should maintain the current API in the gocql package, but move much of the internal things out into sub packages.

Start by reviewing the gocql package and its current public API to identify which internal driver functionality is coupled there. Done means moving appropriate internal components into subpackages while preserving the existing gocql API; the issue does not name specific files or tests.

Written by the indexing model from the issue text.