ColumnFilter::toString doesn't return a valid CQL fragment

This was changed in CASSANDRA-16415 to include indications about queried vs fetched reagular & static columns. However, the result is used by AbstractReadQuery::toCQLString, which causes it to produce an illegal query string.

This breaks a couple of dtests because they're looking for CQL strings in logs, which are no longer found:

• upgrade_tests/paging_test.py::TestPagingWithDeletions::test_failure_threshold_deletions
• cql_test.py::TestCQLSlowQuery has a couple of failing tests, test_local_query/test_remote_query

We should also check audit and fql logs (and any other place where toCQLString is used.