add mutex lock implementation

sync/lock/lock.go

@@ -2,9 +2,14 @@
 package lock
 
 import (
+	"errors"
 	"time"
 )
 
+var (
+	ErrLockTimeout = errors.New("lock timeout")
+)
+
 // Lock is a distributed locking interface
 type Lock interface {
 	// Acquire a lock with given id