server/*: refactor AllocPeer

[disksing]

Signed-off-by: disksing i@disksing.com

What problem does this PR solve?

IDs may be wasted because AllocPeer is called too early. See #1924

Now we use operator.Builder to create operators. Since it supports allocating ID automatically, we can defer ID allocation to Builder.

What is changed and how it works?

• allocate IDs in builder
• change cluster.AllocPeer to cluster.AllocID. (some tests keeps using AllocPeer)

Check List

Tests

• Unit test

[codecov-io]

Codecov Report

Merging #2073 into master will increase coverage by 0.2%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #2073     +/-   ##
=========================================
+ Coverage   76.77%   76.98%   +0.2%     
=========================================
  Files         191      191             
  Lines       19332    19294     -38     
=========================================
+ Hits        14842    14853     +11     
+ Misses       3368     3339     -29     
+ Partials     1122     1102     -20

Impacted Files	Coverage Δ
server/cluster/cluster.go	81.54% <ø> (+0.38%)	⬆️
server/schedule/operator/operator.go	95.91% <ø> (ø)	⬆️
server/schedule/region_scatterer.go	82.6% <100%> (+1.75%)	⬆️
server/schedulers/adjacent_region.go	76.47% <100%> (+1.99%)	⬆️
server/schedule/operator/builder.go	95.12% <100%> (ø)	⬆️
server/schedulers/shuffle_hot_region.go	69.79% <100%> (+2.11%)	⬆️
server/schedulers/hot_region.go	86.39% <100%> (+0.64%)	⬆️
server/schedule/checker/replica_checker.go	83.33% <100%> (+2.96%)	⬆️
server/handler.go	51.61% <100%> (+0.76%)	⬆️
server/schedulers/shuffle_region.go	84.28% <100%> (+3.2%)	⬆️
... and 8 more

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[lhy1024]

LGTM

[rleungx]

LGTM

[disksing]

/merge

[sre-bot]

/run-all-tests

[disksing]

CI problem. re-run.

[disksing]

/merge

[lhy1024]

/merge

[lhy1024]

/run-all-tests