simulator: make heartbeat more reasonable

[rleungx]

What problem does this PR solve?

Currently, if one store is going to send the heartbeat, it needs to wait until the previous one finishes it.

What is changed and how it works?

This PR tries to use different goroutines to do these jobs.

Check List

Tests

• Manual test (add detailed scripts or steps below)

[codecov-io]

Codecov Report

Merging #1418 into master will decrease coverage by 0.47%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1418      +/-   ##
==========================================
- Coverage   68.01%   67.53%   -0.48%     
==========================================
  Files         158      158              
  Lines       15208    15261      +53     
==========================================
- Hits        10344    10307      -37     
- Misses       3950     4025      +75     
- Partials      914      929      +15

Impacted Files	Coverage Δ
tools/pd-simulator/simulator/drive.go	0% <0%> (ø)	⬆️
tools/pd-simulator/simulator/raft.go	14.82% <0%> (-0.71%)	⬇️
tools/pd-simulator/simulator/task.go	0% <0%> (ø)	⬆️
tools/pd-simulator/simulator/statistics.go	0% <0%> (ø)	⬆️
tools/pd-simulator/simulator/node.go	0% <0%> (ø)	⬆️
pkg/tempurl/tempurl.go	57.14% <0%> (-14.29%)	⬇️
server/etcd_kv.go	63.63% <0%> (-10.91%)	⬇️
server/leader.go	80.25% <0%> (-7.15%)	⬇️
server/namespace_cluster.go	84.41% <0%> (-3.9%)	⬇️
server/util.go	70.27% <0%> (-2.71%)	⬇️
... and 6 more

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[rleungx]

/rebuild