Fix: fix rand.Seed() duplicate concurrent calls

[CoolIceV]

What this PR does:
Move the rand.Seed() call to init()

Which issue(s) this PR fixes:
Fixes #1957

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[binbin0325]

rand.Seed only need to initialize once

[justxuewei]

Yes, I think there should be a sync.Once to initialize the seed globally. Did dubbo-go have it before?

[CoolIceV]

Each init() is called only once in golang, and I think it is sufficient to ensure that the seed does not change after the program starts running, regardless of the number of times the rand.Seed() is called in the program.

In addition, the imported third-party package may call rand.Seed() even if we only run it once

[justxuewei]

Well, the number of initializations of the rand seed is not a very strict limit. This PR looks good for me. BTW, I am saying that initialize the rand seed globally, instead of initialize that in init() function of each package, but it doesn't matter now.

[codecov-commenter]

Codecov Report

Merging #1958 (ed6feb5) into 3.0 (026711b) will decrease coverage by 0.60%.
The diff coverage is 27.27%.

@@            Coverage Diff             @@
##              3.0    #1958      +/-   ##
==========================================
- Coverage   44.96%   44.35%   -0.61%     
==========================================
  Files         287      283       -4     
  Lines       17135    17023     -112     
==========================================
- Hits         7704     7551     -153     
- Misses       8618     8668      +50     
+ Partials      813      804       -9     

Impacted Files	Coverage Δ
cluster/cluster/base/cluster_invoker.go	24.44% <ø> (ø)
cluster/cluster/broadcast/cluster_invoker.go	80.00% <ø> (ø)
cluster/cluster/failback/cluster_invoker.go	75.82% <ø> (ø)
cluster/cluster/failover/cluster_invoker.go	66.66% <ø> (ø)
cluster/cluster/failsafe/cluster_invoker.go	81.81% <ø> (ø)
cluster/cluster/forking/cluster_invoker.go	58.33% <ø> (ø)
cluster/loadbalance/ringhash/ringhash.go	63.33% <ø> (ø)
cluster/router/tag/match.go	90.69% <ø> (ø)
cluster/router/tag/router.go	59.64% <ø> (ø)
common/extension/proxy_factory.go	0.00% <ø> (ø)
... and 131 more

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