Don't re-open frozen segments

[mh0lt]

At the moment

func (s *RoSnapshots) rebuildSegments(fileNames []string, open bool, optimistic bool) error

Reopens all segments even if they are already open and frozen, hence will not change due to extract and merge operations.

This is inefficient as it closes and open files which costs os resources and potentially disturbs file caching.

[stevemilk]

During snapshotsync, part of downloaded files can be frozen, so we need to reopen in case files are partially downloaded. Hence I think question goes back to how to verify the integrity of files.
It occurred to me that we can add a flag: downloadComplete and set it to be true after snapshotSync complete downloading so that subsequent reopen will not open those opened fils. The only flaw is it has to wait all files complete downloading.
I draft a PR and you can check if it's viable #12307.

btw, I prepared #12245 for this kind of issue too, need to pass the integration tests first though(i'm following up). This intends to maintain commitment files for downloaded files.
hope to get your advice, thank you :)

[AskAlexSharov]

done by #12332