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/server-main.go

@@ -36,7 +36,7 @@ import (
 func init() {
 	logger.Init(GOPATH, GOROOT)
 	logger.RegisterUIError(fmtError)
-	gob.Register(HashMismatchError{})
+	gob.Register(VerifyFileError(""))
 	gob.Register(DeleteFileError(""))
 }