tolerate 2 replicas failure with 3 data replicas

[hicqu]

Feature Request

Currently if we want to tolerate 2 replicas failure, TiDB clusters need to be deployed with 5 data replicas. It's too expensive for some users. However, some other databases can use only 3 data replicas to reach this. Support this feature can make TiDB be better than them.

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

Describe the feature you'd like:

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

We can image that mainly changes about the feature will be in TiKV, especially component Raft.
And there are some articles described the same problem, and tried to resolve it. One solution is based on Witness, a new role in Raft.

[zhangjinpeng87]

How to provide service when 2 replicas failed?