dgraph-io / badger / 3577

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Use fastRand instead of locked-rand in skiplist (#1173)

The math/rand package (https://golang.org/src/math/rand/rand.go) uses
a global lock to allow concurrent access to the rand object.

The PR replaces `math.Rand` with `ristretto/z.FastRand()`. `FastRand`
is much faster than `math.Rand` and `rand.New(..)` generators.

The change improves concurrent writes to skiplist by ~30%
```go
func BenchmarkWrite(b *testing.B) {
	value := newValue(123)
	l := NewSkiplist(int64((b.N + 1) * MaxNodeSize))
	defer l.DecrRef()
	b.ResetTimer()
	b.RunParallel(func(pb *testing.PB) {
		rng := rand.New(rand.NewSource(time.Now().UnixNano()))
		for pb.Next() {
			l.Put(randomKey(rng), y.ValueStruct{Value: value, Meta: 0, UserMeta: 0})
		}
	})
}
```
```
name      old time/op  new time/op  delta
Write-16   657ns ± 3%   441ns ± 1%  -32.94%  (p=0.000 n=9+10)
```

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