messageID: optimize NewSafeID using atomic instead of mutex lock

name                  old time/op    new time/op    delta
NewSafeID-20            13.9ns ± 2%     7.5ns ± 1%  -46.16%  (p=0.008 n=5+5)
NewSafeIDParallel-20    24.2ns ± 6%    22.1ns ± 1%   -9.06%  (p=0.008 n=5+5)

name                  old alloc/op   new alloc/op   delta
NewSafeID-20             0.00B          0.00B          ~     (all equal)
NewSafeIDParallel-20     8.00B ± 0%     8.00B ± 0%     ~     (all equal)

name                  old allocs/op  new allocs/op  delta
NewSafeID-20              0.00           0.00          ~     (all equal)
NewSafeIDParallel-20      1.00 ± 0%      1.00 ± 0%     ~     (all equal)

Signed-off-by: Koichi Shiraishi <zchee.io@gmail.com>

messageID.go

@@ -1,6 +1,6 @@
 package slack
 
-import "sync"
+import "sync/atomic"
 
 // IDGenerator provides an interface for generating integer ID values.
 type IDGenerator interface {
@@ -11,20 +11,17 @@ type IDGenerator interface {
 // concurrent use by multiple goroutines.
 func NewSafeID(startID int) IDGenerator {
 	return &safeID{
-		nextID: startID,
-		mutex:  &sync.Mutex{},
+		nextID: int64(startID),
 	}
 }
 
 type safeID struct {
-	nextID int
-	mutex  *sync.Mutex
+	nextID int64
 }
 
-func (s *safeID) Next() int {
-	s.mutex.Lock()
-	defer s.mutex.Unlock()
-	id := s.nextID
-	s.nextID++
+func (s *safeID) Next() (id int) {
+	id = int(s.nextID)
+	atomic.AddInt64(&s.nextID, 1)
+
 	return id
 }