Bring in safe mode support (#8478)

This PR refactors object layer handling such
that upon failure in sub-system initialization
server reaches a stage of safe-mode operation
wherein only certain API operations are enabled
and available.

This allows for fixing many scenarios such as

 - incorrect configuration in vault, etcd,
   notification targets
 - missing files, incomplete config migrations
   unable to read encrypted content etc
 - any other issues related to notification,
   policies, lifecycle etc

cmd/metrics.go

@@ -83,9 +83,9 @@ func (c *minioCollector) Collect(ch chan<- prometheus.Metric) {
 	minioVersionInfo.WithLabelValues(Version, CommitID).Set(float64(1.0))
 
 	// Fetch disk space info
-	objLayer := globalObjectAPI
+	objLayer := newObjectLayerFn()
 	// Service not initialized yet
-	if objLayer == nil || globalSafeMode {
+	if objLayer == nil {
 		return
 	}