Move Go() promise-like func from utils to pkg/promise

This is the first of two steps to break the archive package's dependence
on utils so that archive may be moved into pkg.  Also, the `Go()`
function is small, concise, and not specific to the docker internals, so
it is a good candidate for pkg.

Signed-off-by: Rafe Colton <rafael.colton@gmail.com>
Upstream-commit: b845a62149d5f4990462ac6c9167c5cfaa0e66cb
Component: engine

components/engine/daemon/attach.go

@@ -10,6 +10,7 @@ import (
 	"github.com/docker/docker/pkg/ioutils"
 	"github.com/docker/docker/pkg/jsonlog"
 	"github.com/docker/docker/pkg/log"
+	"github.com/docker/docker/pkg/promise"
 	"github.com/docker/docker/utils"
 )
 
@@ -246,7 +247,7 @@ func (daemon *Daemon) Attach(streamConfig *StreamConfig, openStdin, stdinOnce, t
 		}()
 	}
 
-	return utils.Go(func() error {
+	return promise.Go(func() error {
 		defer func() {
 			if cStdout != nil {
 				cStdout.Close()