vinyl: don't throttle DDL

Since DDL is triggered by the admin, it can be deliberately initiated
when the workload is known to be low. Throttling it along with DML
requests would only cause exasperation in this case. So we don't apply
disk-based rate limit to DDL. This should be fine, because the
disk-based limit is set rather strictly to let the workload some space
to grow, see vy_regulator_update_rate_limit(), and in contrast to the
memory-based limit, exceeding the disk-based limit doesn't result in
abrupt stalls - it may only lead to a gradual accumulation of disk space
usage and read latency.

Closes #4238

(cherry picked from commit 6a44ac62)