Use goroutine-safe version of rand.Source

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

random/random.go

@@ -0,0 +1,34 @@
+package random
+
+import (
+	"math/rand"
+	"sync"
+	"time"
+)
+
+// copypaste from standard math/rand
+type lockedSource struct {
+	lk  sync.Mutex
+	src rand.Source
+}
+
+func (r *lockedSource) Int63() (n int64) {
+	r.lk.Lock()
+	n = r.src.Int63()
+	r.lk.Unlock()
+	return
+}
+
+func (r *lockedSource) Seed(seed int64) {
+	r.lk.Lock()
+	r.src.Seed(seed)
+	r.lk.Unlock()
+}
+
+// NewSource returns math/rand.Source safe for concurrent use and initialized
+// with current unix-nano timestamp
+func NewSource() rand.Source {
+	return &lockedSource{
+		src: rand.NewSource(time.Now().UnixNano()),
+	}
+}