fs/cache: add --fs-cache-expire-duration to control the fs cache

This commit makes the previously statically configured fs cache configurable.

It introduces two parameters `--fs-cache-expire-duration` and
`--fs-cache-expire-interval` to control the caching of the items.

It also adds new interfaces to lib/cache to set these.

fs/config.go

@@ -123,6 +123,8 @@ type ConfigInfo struct {
 	RefreshTimes           bool
 	NoConsole              bool
 	TrafficClass           uint8
+	FsCacheExpireDuration  time.Duration
+	FsCacheExpireInterval  time.Duration
 }
 
 // NewConfig creates a new config with everything set to the default
@@ -160,6 +162,8 @@ func NewConfig() *ConfigInfo {
 	c.MultiThreadStreams = 4
 
 	c.TrackRenamesStrategy = "hash"
+	c.FsCacheExpireDuration = 300 * time.Second
+	c.FsCacheExpireInterval = 60 * time.Second
 
 	return c
 }