Use mark and sweep for exec command removal

This takes the final removal for exec commands in two steps.  The first
GC tick will mark the exec commands for removal and then the second tick
will remove the config from the daemon.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

daemon/exec.go

@@ -28,6 +28,7 @@ type execConfig struct {
 	OpenStderr bool
 	OpenStdout bool
 	Container  *Container
+	canRemove  bool
 }
 
 type execStore struct {
@@ -256,12 +257,15 @@ func (d *Daemon) execCommandGC() {
 		var (
 			cleaned          int
 			liveExecCommands = d.containerExecIds()
-			ids              = d.execCommands.List()
 		)
-		for _, id := range ids {
-			if _, exists := liveExecCommands[id]; !exists {
+		for id, config := range d.execCommands.s {
+			if config.canRemove {
 				cleaned++
 				d.execCommands.Delete(id)
+			} else {
+				if _, exists := liveExecCommands[id]; !exists {
+					config.canRemove = true
+				}
 			}
 		}
 		logrus.Debugf("clean %d unused exec commands", cleaned)