scanner: Add a config to disable short sleep between objects scan (#18734)

Add a hidden configuration under the scanner sub section to configure if
the scanner should sleep between two objects scan. The configuration has
only effect when there is no drive activity related to s3 requests or
healing.

By default, the code will keep the current behavior which is doing
sleep between objects.

To forcefully enable the full scan speed in idle mode, you can do this:

   `mc admin config set myminio scanner idle_speed=full`

internal/config/config.go

@@ -274,7 +274,7 @@ type KV struct {
 	Key   string `json:"key"`
 	Value string `json:"value"`
 
-	Deprecated bool `json:"-"`
+	HiddenIfEmpty bool `json:"-"`
 }
 
 func (kv KV) String() string {
@@ -1447,7 +1447,7 @@ func (cs *SubsysInfo) WriteTo(b *strings.Builder, off bool) {
 			continue
 		}
 		// Ignore empty and deprecated values
-		if dkv.Deprecated && kv.Value == "" {
+		if dkv.HiddenIfEmpty && kv.Value == "" {
 			continue
 		}
 		// Do not need to print if state is on