control: formatted duration for config trustengine.cache.expiration

The `trustengine.cache.expiration` configuration option would accept
durations as number of nanoseconds. Most likely, this was accidental,
all other configuration options for durations accept formatted duration
strings, with unit suffix.

Change `trustengine.cache.expiration` to accept (only) formatted
duration strings.

This is a potentially compatibility breaking change for existing control
service configuration files.
The `trustengine.cache` configuration block is marked as experimental
and is likely not widely used, so no transition mechanism is added.

doc/manuals/control.rst

@@ -311,11 +311,9 @@ considers the following options.
 
       Disable caching entirely.
 
-   .. option:: trustengine.cache.expiration = <int> (Default: 60000000000)
+   .. option:: trustengine.cache.expiration = <duration> (Default: "1m")
 
-      Expiration of cached entries in nanoseconds.
-
-      **TODO:** this should be changed to accept values in :ref:`duration format <control-conf-duration>`.
+      Expiration time for cached entries.
 
 .. object:: drkey
 

private/trust/config/config.go

@@ -20,6 +20,7 @@ import (
 
 	"github.com/patrickmn/go-cache"
 
+	"github.com/scionproto/scion/pkg/private/util"
 	"github.com/scionproto/scion/private/config"
 )
 
@@ -47,20 +48,20 @@ func (cfg *Config) ConfigName() string {
 }
 
 type Cache struct {
-	Disable    bool          `toml:"disable,omitempty"`
-	Expiration time.Duration `toml:"expiration,omitempty"`
+	Disable    bool         `toml:"disable,omitempty"`
+	Expiration util.DurWrap `toml:"expiration,omitempty"`
 }
 
 func (cfg *Cache) New() *cache.Cache {
 	if cfg.Disable {
 		return nil
 	}
-	return cache.New(cfg.Expiration, time.Minute)
+	return cache.New(cfg.Expiration.Duration, time.Minute)
 }
 
 func (cfg *Cache) InitDefaults() {
-	if cfg.Expiration == 0 {
-		cfg.Expiration = defaultExpiration
+	if cfg.Expiration.Duration == 0 {
+		cfg.Expiration.Duration = defaultExpiration
 	}
 }