Use goroutine-safe version of rand.Source

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

random/random_test.go

@@ -0,0 +1,22 @@
+package random
+
+import (
+	"math/rand"
+	"sync"
+	"testing"
+)
+
+// for go test -v -race
+func TestConcurrency(t *testing.T) {
+	rnd := rand.New(NewSource())
+	var wg sync.WaitGroup
+
+	for i := 0; i < 10; i++ {
+		wg.Add(1)
+		go func() {
+			rnd.Int63()
+			wg.Done()
+		}()
+	}
+	wg.Wait()
+}