cql3: hide virtual columns from WRITETIME() and TTL()
cql3: hide virtual columns from WRITETIME() and TTL()

Virtual columns should not be visible to the user,
so they are now hidden not only from directly selecting them,
but also via WRITETIME() and TTL() keywords.

Fixes #4288

scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla