Support syncing when disk is very slow

[lance6716]

Feature Request

Is your feature request related to a problem? Please describe:

A user deploy DM in an environment that has very slow disk. that has help revealing some BUG of DM such as

• load phase of all mode may lost data when worker frequently starts/stops task #1377
• worker recieved a bound watch, but failed to read bound information in etcd and didn't retry or kill itself
• query-status shows nothing, while can't add task because of already exists (not enough information in log)

Describe the feature you'd like:

• expose more etcd error and metrics (already in handle etcd error from Etcd.Err() #1219, export some metrics for embed etcd #1218), and warn when disk is bad
• test DM in slow disk

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[lance6716]

I have used chaosmesh to try imitate a bad disk environment, but not effective to reveal bugs. We might try use failpoint with percent probability to inject into etcd API, and check if it will cause inconsistency in DM.

@zeminzhou

[lance6716]

(removed the BUG label because we need further investigating if it's has been fixed)