DDL is blocked if the MDL info in ETCD is expired

[wjhuang2016]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

During a DDL, a TiDB server writes the MDL info to ETCD with a lease. If this TiDB server can't connect to PD for a long time, the MDL info is expired.
If so, the DDL is blocked.

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

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

master