[server] RemoteLogFetcher optimize to async downloading

[swuferhong]

Search before asking

• I searched in the issues and found nothing similar.

Motivation

Currently, RemoteLogFetcher fetch remote log sync which will cause the recovery process may be significantly slowed down by the download time of remote segments, especially when there are many segments to fetch. See:#2855 (comment)

Solution

No response

Anything else?

No response

Willingness to contribute

• I'm willing to submit a PR!

[fresh-borzoni]

I might help here, since I spent some time optimising this in rust SDK :)