It's now named `execute_thrift_schema_command` in `query_processor`.
This allows us to remove yet another
`query_processor::get_migration_manager()` call.
Now that `execute_thrift_schema_command` sits near
`execute_schema_statement` (the latter used for CQL), we can see a
certain similarity. The Thrift version should also in theory get a retry
loop like the one CQL has, so the similarity would become even stronger.
Perhaps the two functions could be refactored to deduplicate some logic
later.