scheduler: fix region scatter may transfer leader to removed peer

[nolouch]

Signed-off-by: nolouch nolouch@gmail.com

What problem does this PR solve?

Somtimes region scatter may timeout, it cause by:

• Leader already changed but scatter-region step still use old leader
• Leader may transfer to a removed peer

log like:

[region 2109921] operator timeout: scatter-region (kind:leader,region,admin, region:2109921(5233,677), createAt:2019-03-26 14:29:14.885583251 +0800 CST m=+1888726.900848985, currentStep:10, steps:[add learner peer 2109925 on store 5 promote learner peer 2109925 on store 5 to voter remove peer on store 8 transfer leader from store 0 to store 5 add learner peer 2109926 on store 6 promote learner peer 2109926 on store 6 to voter remove peer on store 4 transfer leader from store 0 to store 6 add learner peer 2109927 on store 1 promote learner peer 2109927 on store 1 to voter transfer leader from store 7 to store 8 remove peer on store 7 transfer leader from store 0 to store 1]) timeout

add  store 5
remove store 8
transfer to 5
add store 6
remove store 4
transfer to 6
add  store 1 
transfer from 7 to 8
remove  store 7
transfer to 1

the origin leader is on store 7.

What is changed and how it works?

Fix the operator create logic.

Check List

Tests

• Unit test

[disksing]

@nolouch The log you provided does not transfer leader to removed peer. The steps are:

- add 5
- transfer 2
- remove 1
- transfer 5
- add 4
- remove 2
- transfer 4

[nolouch]

@disksing I update the log, this one is pick from test cluster.

[rleungx]

@nolouch CI failed.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@56f4100). Click here to learn what that means.
The diff coverage is 78.02%.

@@           Coverage Diff            @@
##             master   #1482   +/-   ##
========================================
  Coverage          ?   67.4%           
========================================
  Files             ?     158           
  Lines             ?   15454           
  Branches          ?       0           
========================================
  Hits              ?   10417           
  Misses            ?    4092           
  Partials          ?     945

Impacted Files	Coverage Δ
server/grpc_service.go	53.75% <0%> (ø)
server/schedule/mockcluster.go	83.41% <0%> (ø)
server/handler.go	55.58% <33.33%> (ø)
server/schedule/operator.go	86.26% <68.42%> (ø)
server/schedule/region_scatterer.go	88.14% <90.47%> (ø)

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[nolouch]

PTAL @disksing @rleungx

[rleungx]

How about using a variable for origin.GetLeader().GetStoreId()

[rleungx]

It's better to add some comments to explain why we put these steps in the end.