Don't let user drop (or generally break) tables in system_distributed

There is not currently no particular restrictions on schema modifications to tables of the system_distributed keyspace. This does mean you can drop those tables, or even alter them in wrong ways like dropping or renaming columns. All of which is guaranteed to break stuffs (that is, repair if you mess up with on of it's table, or MVs if you mess up with view_build_status).

I'm pretty sure this was never intended and is an oversight of the condition on ALTERABLE_SYSTEM_KEYSPACES in ClientState. That condition is such that any keyspace not listed in ALTERABLE_SYSTEM_KEYSPACES (which happens to be the case for system_distributed) has no specific restrictions whatsoever, while given the naming it's fair to assume the intention that exactly the opposite.