Configurable Replica Read Timeout with Retry Feature Request

Is your feature request related to a problem? Please describe:
One of common problems running TiDB in the cloud on network attached disks (Amazon EBS, Google PD or Azure managed disks) is temporary elevated disk IO latency. This can happen if a cloud provider storage node fails and goes through a repair procedure. During the repair phase a network attached disk exhibits 100ms or even single digit second latency vs single digit millisecond latency under the normal conditions.

Describe the feature you'd like:
If TiDB customer uses Follower Read or Stale Read feature, it is possible to retry a request initially landed on the TiKV node with network disk exhibiting elevated latency on the other TiKV replica. While retry policy already exists in tikv go-client, the default network timeout is 10s of seconds.

OLTP workload on TiDB could leverage an introduction of system variable tidb_tikv_read_timeout which then could be passed as a context timeout for TiKV requests made by TiDB layer and rely on existing selector logic to retry requests on other replicas. The implementation of this feature needs also to take care of the following:

• The intermediate timeouts should still bubble up in metrics
• Back-off should be skipped on this timeout
• Right now Stale Read retry always goes to the leader (avoid dataIsNotReady error while retrying stale read on the leader tikv/client-go#765). This needs to be changed so that it retries on leader only for DataIsNot ready error, but after timeout it should go to another replica.

Describe alternatives you've considered:
TiDB already has a max_execution_timeout system variable, but it is not used as a context deadline in go-client to network calls from TiDB to TiKV. Moreover, if TiKV request takes longer than max_execution_timeout, then the session is marked as killed and retry won’t happen.

Teachability, Documentation, Adoption, Migration Strategy:
The feature would be fully controlled by session variable tidb_tikv_read_timeout.