feat: Single drive XL implementation (#14970)

Main motivation is move towards a common backend format
for all different types of modes in MinIO, allowing for
a simpler code and predictable behavior across all features.

This PR also brings features such as versioning, replication,
transitioning to single drive setups.

cmd/storage-errors.go

@@ -116,6 +116,12 @@ var errDoneForNow = errors.New("done for now")
 // to proceed to next entry.
 var errSkipFile = errors.New("skip this file")
 
+// Returned by FS drive mode when a fresh disk is specified.
+var errFreshDisk = errors.New("FS backend requires existing disk")
+
+// errXLBackend XL drive mode requires fresh deployment.
+var errXLBackend = errors.New("XL backend requires fresh disk")
+
 // StorageErr represents error generated by xlStorage call.
 type StorageErr string