Don't invalid region in tryFollowers when set user-defined req timeout and meet DeadLineExceededErr

[crazycs520]

Issue

1. Minimal reproduce step

In the following case, suppose the region leader is in store1.

	ca = replicaSelectorAccessPathCase{
		reqType:   tikvrpc.CmdGet,
		readType:  kv.ReplicaReadMixed,
		staleRead: true,
		timeout:   time.Second,
		label:     nil,
		accessErr: []RegionErrorType{DeadLineExceededErr, ServerIsBusyErr, ServerIsBusyErr},

2. What did you expect to see?

No backoff.

		expect: &accessPathResult{
			accessPath: []string{
				"{addr: store1, replica-read: false, stale-read: true}",
				"{addr: store2, replica-read: true, stale-read: false}",
				"{addr: store3, replica-read: true, stale-read: false}"},
			regionIsValid:   true,
		},

3. What did you see instead?

		expect: &accessPathResult{
			...
			regionIsValid:   false,
		},

The request has user-defined timeout 1s, the replica selector logic is:

1. try store1 by stale-read, but got DeadLineExceededErr.
2. retry store2, got ServerIsBusyErr.
3. retry store3, got ServerIsBusyErr too, and then invalidate the region in cache.

The expected result is doesn't invalidate region in cache, and the do fast retry without user-defined timeout.