Investigate whether to keep Sled or switch to something else

[abustany]

We're currently using https://github.com/spacejam/sled/ for storage, which is pure Rust (makes compiling easy) and nice to use. Whether it is a good choice going forward regarding reliability and forward compatibility is maybe more arguable... Investigate whether switching to something like LevelDB (or SQLite with a trivial Key/Value schema?) makes sense. Alternatively, switching to SQLite but with a more structured schema would make features like "sort by XXX" trivial to implement.