Fix runaway allocation on /v2/_catalog

Introduced a Catalog entry in the configuration struct. With it,
it's possible to control the maximum amount of entries returned
by /v2/catalog (`GetCatalog` in registry/handlers/catalog.go).

It's set to a default value of 1000.

`GetCatalog` returns 100 entries by default if no `n` is
provided. When provided it will be validated to be between `0`
and `MaxEntries` defined in Configuration. When `n` is outside
the aforementioned boundary, an error response is returned.

`GetCatalog` now handles `n=0` gracefully with an empty response
as well.

Signed-off-by: José D. Gómez R. <1josegomezr@gmail.com>

configuration/configuration_test.go

@@ -70,6 +70,9 @@ var configStruct = Configuration{
 			},
 		},
 	},
+	Catalog: Catalog{
+		MaxEntries: 1000,
+	},
 	HTTP: struct {
 		Addr         string        `yaml:"addr,omitempty"`
 		Net          string        `yaml:"net,omitempty"`
@@ -521,6 +524,7 @@ func copyConfig(config Configuration) *Configuration {
 	configCopy.Version = MajorMinorVersion(config.Version.Major(), config.Version.Minor())
 	configCopy.Loglevel = config.Loglevel
 	configCopy.Log = config.Log
+	configCopy.Catalog = config.Catalog
 	configCopy.Log.Fields = make(map[string]interface{}, len(config.Log.Fields))
 	for k, v := range config.Log.Fields {
 		configCopy.Log.Fields[k] = v