stats: reduce stats collecor's lock contention

[alivxxx]

What problem does this PR solve?

We will create a session stats collector for each session and link them into a list. So when there are many sessions, it will take longer to loop the list, and holding the list head's lock during the whole process will block other operations, like NewSessionStatsCollector.

What is changed and how it works?

When looping the list, only holds the lock of necessary stats collector, that is the previous one and current. So it will only holds at most two locks at the same time.

Check List

Tests

• Unit test

Code changes

• Has exported function/method change

Side effects

• None

Related changes

• None

[codecov-io]

Codecov Report

Merging #9233 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #9233      +/-   ##
=========================================
- Coverage    67.3%   67.3%   -0.01%     
=========================================
  Files         373     373              
  Lines       78449   78441       -8     
=========================================
- Hits        52803   52795       -8     
+ Misses      20911   20910       -1     
- Partials     4735    4736       +1

Impacted Files	Coverage Δ
statistics/update.go	83.69% <100%> (-0.28%)	⬇️
ddl/session_pool.go	82.75% <0%> (-10.35%)	⬇️
ddl/delete_range.go	78.83% <0%> (-0.53%)	⬇️
store/tikv/scan.go	77.31% <0%> (+3.36%)	⬆️

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[winoros]

lgtm

[zz-jason]

LGTM

[zz-jason]

/run-all-tests