schema cache should try to correct previous wrong timestamp

[hihihuhu]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

when get schema encounters transient error when gets its timestamp, the timestamp of the schema is zero
https://github.com/pingcap/tidb/blob/v7.3.0/domain/domain.go#L223
it makes the cache useless because the timestamp is zero and it won't recover because the next success reload won't correct the timestamp until another ddl happens, the reload will exit at without correct the timestamp
https://github.com/pingcap/tidb/blob/v7.3.0/domain/domain.go#L227

2. What did you expect to see? (Required)

transient error should not prevent schema cache being useful forever

3. What did you see instead (Required)

schema cache is not useable

4. What is your TiDB version? (Required)

master