Introduce more fine grained schema/DDL lease refresh interval

## Enhancement

When TiDB runs on attached disks in cloud, some of disks could exhibit long latency for a minute duration. In order to avoid schema lease expiration in such cases when system metadata can't be served in time, we have increased https://docs.pingcap.com/tidb/stable/tidb-configuration-file/#lease to 5 minutes. With this changes the lease refresh runs only each 2-3 minutes: https://github.com/pingcap/tidb/blob/eaa8c332711311fe6068a7cc8dc625f880797e81/pkg/domain/domain.go#L1094-L1096 as it is configured to run half of lease interval.

For this case we would prefer to keep lease refresh interval to 30 seconds (or 1/10 of the lease), so that TiDB can have a higher chance to refresh schema on time.

This approach can also help when there is a hot key collocated on the same node as system metadata or there are other problem with schema lease like cache misses due to stale reads.