It seems some opened files are not closed

[baurine]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

Run tiup-cluster UI for a long time ( related PR: #706 )

2. What did you expect to see?

Works well all time.

3. What did you see instead?

It becomes abnormal after a long time. It says some files too many open files.

4. What version of TiUP are you using (tiup --version)?

mater branch after v1.2.0