vinyl: rename vy_log_record::index_id/space_id to index_def_id/space_def_id

I'm planning to assign a unique identifier to each vinyl index so that
it could be used instead of lsn for identifying indexes in vylog. In
order not to confuse it with the index ordinal number, let's rename
vy_log_record::index_id to index_def_id and, for consistency, space_id
to space_def_id.