Cache indexes for "large" files so that startup is faster

[tstack]

It can take quite a while to index a large set of files, so we might want to cache the index on disk for "big" files. Hopefully, that should speed up restart time.

--- Want to back this issue? **[Post a bounty on it!](https://www.bountysource.com/issues/10776135-cache-indexes-for-large-files-so-that-startup-is-faster?utm_campaign=plugin&utm_content=tracker%2F449456&utm_medium=issues&utm_source=github)** We accept bounties via [Bountysource](https://www.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F449456&utm_medium=issues&utm_source=github).

[cicku]

I may find a bug here, will send info later.

[f9z]

Yes it'd be great to save indices