Don't backoff on send failure if it will access a different store in the next time

[sticnarf]

If sending RPC fails, we always backoff and wait:

client-go/internal/locate/region_request.go

Line 1310 in 2807409

err = bo.Backoff(retry.BoTiKVRPC, errors.Errorf("send tikv request error: %v, ctx: %v, try next peer later", err, ctx))

However, it is not necessary if we switch to sending to another store. For example, on follower read, in the next retry, we will retry the next follower. Then, it's totally a waste of time to wait for a while.