Cache settings

[nbenaglia]

As user I'd like to decide when cache files in ~/.local/share/gossip/cache/ should be cleaned:

• when gossip is shut down
• when cache files are older than N days

In settings the "Database" tab should be renamed in "Storage" (more generic) and the above new cache options may stay together with the current database tab content .

[mikedilger]

Some cached files are heavily used (your own avatar) and others haven't been used in a long time. But we have no reliable way to distinguish between these cases.

Many people's filesystems don't maintain or support last-accessed times because every read then requires a write. So we cannot reliably ask "only delete files that haven't been accessed recently."

If we delete cached files when they get old, then people will be almost immediately re-download some of those.

Options:

1. Try using the file system access time anyways, fall back to modification time. Redownloads will happen.
2. Maintain access times in LMDB somewhere. If doing this, we should save the etags there too.
3. Separate avatars and banners from other content, and only prune the other content.

[mikedilger]

Ok I did option 1 for now. I think option 3 is best, but I'll do it later.

[nbenaglia]

I update to track the changes.

Option 1 done with commit 6451709