TiDB may access an unready store

[zyguan]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

see tikv/client-go#1111

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

TiDB should avoid to access an unreachable store.

3. What did you see instead (Required)

TiDB will access an unready store once it registers itself to PD with meta changes.

4. What is your TiDB version? (Required)

master