Allow optionally to disable range caching. (#9908)

The default behavior is to cache each range requested
to cache drive. Add an environment variable
`MINIO_RANGE_CACHE` - when set to off, it disables
range caching and instead downloads entire object
in the background.

Fixes #9870

cmd/config/cache/config.go

@@ -37,6 +37,7 @@ type Config struct {
 	After         int      `json:"after"`
 	WatermarkLow  int      `json:"watermark_low"`
 	WatermarkHigh int      `json:"watermark_high"`
+	Range         bool     `json:"range"`
 }
 
 // UnmarshalJSON - implements JSON unmarshal interface for unmarshalling