Improve context package

You shouldn't have to import both:

  github.com/docker/distribution/context
  golang.org/x/net/context

just to use the distribution tools and implement the distribution interfaces.

By pulling the Context interface from golang.org/x/net/context into the
context package within the distribution project, you no longer have to import
both packages.

Note: You do not have to change anything anywhere else yet! All current uses
of both packages together will still work correctly because the Context
interface from either package is identical.

I've also made some other minor changes:

- Added a RemoteIP function. It's like RemoteAddr but discards the port suffix
- Added `.String()` to the response duration context value so that JSON log
  formatting shows human-parseable duration and not just number of nano-seconds
- Added WithMapContext(...) to the context package. This is a useful function
  so I pulled it out of the main.go in cmd/registry so that it can be used
  elsewhere.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)

context/util.go

@@ -2,14 +2,12 @@ package context
 
 import (
 	"time"
-
-	"golang.org/x/net/context"
 )
 
 // Since looks up key, which should be a time.Time, and returns the duration
 // since that time. If the key is not found, the value returned will be zero.
 // This is helpful when inferring metrics related to context execution times.
-func Since(ctx context.Context, key interface{}) time.Duration {
+func Since(ctx Context, key interface{}) time.Duration {
 	startedAtI := ctx.Value(key)
 	if startedAtI != nil {
 		if startedAt, ok := startedAtI.(time.Time); ok {
@@ -22,7 +20,7 @@ func Since(ctx context.Context, key interface{}) time.Duration {
 
 // GetStringValue returns a string value from the context. The empty string
 // will be returned if not found.
-func GetStringValue(ctx context.Context, key string) (value string) {
+func GetStringValue(ctx Context, key string) (value string) {
 	stringi := ctx.Value(key)
 	if stringi != nil {
 		if valuev, ok := stringi.(string); ok {