schedule: let merge not easy to timeout

[Connor1996]

What problem does this PR solve?

merge operator may timeout easily cause the merge operator timeout span is calculated as LeaderOperatorWaitTime.

func (o *Operator) IsTimeout() bool {
	var timeout bool
	if o.IsFinish() {
		return false
	}
	if o.kind&OpRegion != 0 {
		timeout = time.Since(o.createTime) > RegionOperatorWaitTime(10m)
	} else {
		timeout = time.Since(o.createTime) > LeaderOperatorWaitTime(10s)
	}
	if timeout {
		operatorCounter.WithLabelValues(o.Desc(), "timeout").Inc()
		return true
	}
	return false
}

What is changed and how it works?

For merge schedule, there will be two operators created. One for the source region (op1), the other for the target region (op2). And cause source region will be destroyed and there will be no heartbeat of it anymore, so op1 will never be finished, instead, it will be cleaned up in the PD background thread. So the op2 is the operator to detect when the merge process finished. But op2 has no steps like AddPeer, RemovePeer, so its timeout is always regarded as LeaderOperatorWaitTime.

This PR makes op2 have the same operator kinds as op1.

Check List

• Need to cherry-pick to the release branch

[nolouch]

CI failed. Should cherry pick to 2.1?

[Connor1996]

/rebuild

[rleungx]

Do we need a test?

[Connor1996]

PTAL, replace the test and check operator timeout

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@c0c07dc). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #1495   +/-   ##
=========================================
  Coverage          ?   67.28%           
=========================================
  Files             ?      158           
  Lines             ?    15459           
  Branches          ?        0           
=========================================
  Hits              ?    10401           
  Misses            ?     4107           
  Partials          ?      951

Impacted Files	Coverage Δ
server/schedule/operator.go	86.26% <100%> (ø)

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[zhouqiang-cl]

/rebuild

[zhouqiang-cl]

/rebuild