schema: split _cluster trigger into multiple funcs

_cluster on_replace trigger in alter.cc was a huge multi-screen
function with many indentation levels. It was not too bad in its
old state. But soon it is going to get more complicated as
_cluster will get a new field - 'name'. Its update will require
own on commit and rollback triggers, own checks, errors.

Trying to fit name processing into the old monstrous function
didn't look too tempting, so the trigger now is split into
multiple functions serving update, insert, and delete separately.
At least it helps to reduce the indentation.

Needed for #5029

NO_DOC=refactoring
NO_CHANGELOG=refactoring
NO_TEST=already covered