storage: close idle open FDs via FileManager

[j-c-m]

This helps a lot for memory usage as the OS free's associated buffers/pages wired to the idle files that used to remain open until max files eviction.

Feature closes idle file FDs after a configurable amount of time rather than leaving them open until eviction presure. Default 60s, 0 disables.

rtorrent:

rakshasa/rtorrent#1888

[rakshasa]

This code is rather old, so I don't exactly remember how it works, but we're already evicting stale File entries before opening iirc.

So we shouldn't need to iterate over every single entry here. And likely this part of the code needs to be refactored.

[j-c-m]

It does evict stale entries currently, if it's at the limit. This is more proactive and closes the files when they are idle.

[rakshasa]

You are manually iterating over all entries, while we should already have (or add) a sorted staleness queue for when evicting due to fd limits.

[rakshasa]

Check out the last couple of PR merges as FM now has a last activity cache used during eviction.

This PR might be a bit too aggressive in closing fds, and the cache likely solves a lot of the issues here.