GC was blocked when doing network chaos on TiKV

[shuke987]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. start an instance
2. do a network chaos on TiKV
3. GC blocked until tikv network chaos were removed

root cause：

1. TiDB will get all tikv's confige through internalClient
2. internalClient will keepalive when once called TiKV
3. When tikv host's iptable was changed ( to do a network chaos ), request data was dropped.
4. TiDB blocks at

   tidb/executor/memtable_reader.go

   Line 206 in 2d755a8

   req, err := http.NewRequest(http.MethodGet, url, nil)

sugest fix:

1. add a timeout to internalClient, maybe 2 minute. ( due to the way we simulate network chaos is rarely appearing at customer's env )

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

GC goes on

3. What did you see instead (Required)

no

4. What is your TiDB version? (Required)

v6.5.0