tikv: make region_request_test stable

[lysu]

What problem does this PR solve?

old TestOnSendFailedWithCloseKnownStoreThenUseNewOne test is occasionally failed

[2019-07-30T11:52:47.513Z] FAIL: region_request_test.go:92: testRegionRequestSuite.TestOnSendFailedWithCloseKnownStoreThenUseNewOne
[2019-07-30T11:52:47.513Z]
[2019-07-30T11:52:47.513Z] region_request_test.go:127:
[2019-07-30T11:52:47.513Z]     c.Assert(err, IsNil)
[2019-07-30T11:52:47.513Z] ... value *errors.withStack = [tikv:9002]TiKV server timeout ("[tikv:9002]TiKV server timeout")
[2019-07-30T11:52:47.513Z]

the old test has a problem new store never be a leader and accept the request.

and old assert will be failed to because of async scheduleReload + race/leak slow running.

What is changed and how it works?

elect a new leader to another store when one store fails

Check List

Tests

• Unit test
• Integration test

Code changes

• N/A

Side effects

• N/A

Related changes

• Need to cherry-pick to the release branch

---

This change is 

[lysu]

/run-all-tests

[codecov]

Codecov Report

Merging #11541 into master will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master     #11541   +/-   ##
===========================================
  Coverage   81.2937%   81.2937%           
===========================================
  Files           425        425           
  Lines         91643      91643           
===========================================
  Hits          74500      74500           
  Misses        11788      11788           
  Partials       5355       5355

[jackysp]

LGTM

[zimulala]

LGTM

[ngaut]

LGTM

[sre-bot]

cherry pick to release-3.0 failed