Why use "worker + sync" is fater than "async" about hardlink work?

[ShuiRuTian]

I want to talk about this PR: #7154

For this PR, the mainly change is to

• change function "hardLinkDir" from async to sync
• use a worker to call "hardLinkDir"

I tested the changes and found it's about 2x faster, which is so surprised for me.

But why? I think node should already handle fs related stuff well.

Ping @zkochan ~