Connection score may be negative

[djshow832]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Run a workload and frequently scale TiDB in and out.

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

The connScore of each backend should always be >= 0.

3. What did you see instead (Required)

Sometimes it's negative. See #644

4. What is your version? (Required)

master