Add WithVersion to context and other cleanup

By adding WithVersion to the context package, we can simplify context setup in
the application. This avoids some odd bugs where instantiation order can lead
to missing instance.id or version from log messages.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

context/doc.go

@@ -3,6 +3,19 @@
 // logging relevent request information but this package is not limited to
 // that purpose.
 //
+// The easiest way to get started is to get the background context:
+//
+// 	ctx := context.Background()
+//
+// The returned context should be passed around your application and be the
+// root of all other context instances. If the application has a version, this
+// line should be called before anything else:
+//
+// 	ctx := context.WithVersion(context.Background(), version)
+//
+// The above will store the version in the context and will be available to
+// the logger.
+//
 // Logging
 //
 // The most useful aspect of this package is GetLogger. This function takes