Create Repositories method

This change removes the Catalog Service and replaces it with a more
simplistic Repositories() method for obtaining a catalog of all
repositories.  The Repositories method takes a pre-allocated slice
and fills it up to the size of the slice and returns the amount
filled.  The catalog is returned lexicographically and will start
being filled from the last entry passed to Repositories().  If there
are no more entries to fill, io.EOF will be returned.

Signed-off-by: Patrick Devine <patrick.devine@docker.com>

Conflicts:
	registry/client/repository.go
	registry/handlers/api_test.go

registry/handlers/app.go

@@ -367,9 +367,6 @@ func (app *App) dispatcher(dispatch dispatchFunc) http.Handler {
 		// Add username to request logging
 		context.Context = ctxu.WithLogger(context.Context, ctxu.GetLogger(context.Context, "auth.user.name"))
 
-		catalog := app.registry.Catalog(context)
-		context.Catalog = catalog
-
 		if app.nameRequired(r) {
 			repository, err := app.registry.Repository(context, getName(context))