tso, server, tests: add a test case for tso allocation bug (#2675)

[ti-srebot]

cherry-pick #2675 to release-4.0

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Signed-off-by: JmPotato ghzpotato@gmail.com

What problem does this PR solve?

Previous PR #2665 tries to fix a bug that tso allocation may fail though it's already a PD leader, which lacks of a test case can cover this situation. This pr adds a test case and makes some slight changes than before.

According to these two TiDB jenkins tests, tso may still have some problems when leader changes. I need more logs to figure out why, so this pr also adds some log info when a tso request arrive.

• https://internal.pingcap.net/idc-jenkins/blue/organizations/jenkins/tidb_ghpr_check_2/detail/tidb_ghpr_check_2/42444/pipeline/
• https://internal.pingcap.net/idc-jenkins/blue/organizations/jenkins/tidb_ghpr_check_2/detail/tidb_ghpr_check_2/42068/pipeline/

What is changed and how it works?

• Try to set lease as soon as possible in SyncTimestamp.
• Add some new methods to Server for test purpose.
• Add more logs.

Check List

Tests

• Unit test
• Integration test

Related changes

• PR tso: fix the bug that TSO request may fail at the time of leader changing #2665

Release note

Add more tests and logs for TSO request.

[codecov-commenter]

Codecov Report

Merging #2678 into release-4.0 will increase coverage by 0.00%.
The diff coverage is 76.92%.

@@              Coverage Diff              @@
##           release-4.0    #2678    +/-   ##
=============================================
  Coverage        77.16%   77.17%            
=============================================
  Files              204      205     +1     
  Lines            21940    22232   +292     
=============================================
+ Hits             16931    17157   +226     
- Misses            3724     3773    +49     
- Partials          1285     1302    +17     

Impacted Files	Coverage Δ
server/api/operator.go	45.40% <ø> (ø)
server/schedule/scheduler.go	69.76% <0.00%> (ø)
server/statistics/store_collection.go	59.70% <ø> (+0.44%)	⬆️
server/util.go	46.91% <0.00%> (-3.09%)	⬇️
server/schedule/operator/step.go	65.40% <41.17%> (-9.78%)	⬇️
server/cluster/store_limiter.go	53.70% <46.15%> (ø)
server/api/store.go	66.32% <58.62%> (+0.98%)	⬆️
server/config/config.go	77.30% <62.50%> (-1.62%)	⬇️
server/handler.go	50.32% <71.42%> (-1.29%)	⬇️
server/schedule/operator_controller.go	81.41% <71.42%> (+0.57%)	⬆️
... and 56 more

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[rleungx]

/merge

[ti-srebot]

Your auto merge job has been accepted, waiting for:

• 2669

[ti-srebot]

/run-all-tests