admin/heal: Fix deep healing to heal objects under more conditions (#8321)

- Heal if the part.1 is truncated from its original size
- Heal if the part.1 fails while being verified in between
- Heal if the part.1 fails while being at a certain offset

Other cleanups include make sure to flush the HTTP responses
properly from storage-rest-server, avoid using 'defer' to
improve call latency. 'defer' incurs latency avoid them
in our hot-paths such as storage-rest handlers.

Fixes #8319

cmd/iam.go

@@ -1306,7 +1306,8 @@ func NewIAMSys() *IAMSys {
 	// The default users system
 	var utype UsersSysType
 	switch {
-	case globalServerConfig.LDAPServerConfig.ServerAddr != "":
+	case globalServerConfig != nil &&
+		globalServerConfig.LDAPServerConfig.ServerAddr != "":
 		utype = LDAPUsersSysType
 	default:
 		utype = MinIOUsersSysType