In v4 of scylladb/scylladb#19598 the last commit of the patch was replaced but this change missed merge so submitting it in a separate patch.
In the current patch, the original functions class correctly marks methods as const where appropriate, and the instance() method now returns a const object. This ensures protection against accidental modifications, as all changes must go through the change_batch object.
Since the functions_changer class was intended to serve the same purpose, it is now redundant. Therefore, we are reverting the commit that introduced it.
Relates scylladb/scylladb#19153Closesscylladb/scylladb#19647
* github.com:scylladb/scylladb:
cql3: functions: replace template with std::function in with_udf_iter()
cql3: functions: improve functions class constness handling
Revert "cql3: functions: make modification functions accessible only via batch class"
4571262e46