backoff: remove global rand.Seed and using locked rand.Rand

[zchee]

rand.Seed will affect globally. Remove it, and implements mutex locked rand.Rand similar of stdlib.

[sethvargo]

What's the reason for the mutex?

[zchee]

@sethvargo Just mimic

• https://github.com/golang/go/blob/go1.19.4/src/math/rand/rand.go#L380-L419

math/rand's globalRand is here:

• https://github.com/golang/go/blob/go1.19.4/src/math/rand/rand.go#L293

math/rand.New returns not mutex locked *rand.Rand

• https://github.com/golang/go/blob/go1.19.4/src/math/rand/rand.go#L66-L69

[zchee]

Q: Should I use a global variable similar to stdlib globalRand?

• https://github.com/golang/go/blob/go1.19.4/src/math/rand/rand.go#L296

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implemented

[zchee]

@sethvargo PTAL