Call `save()` asynchronously for performance

[delphinus]

ref #130

[delphinus]

This is achieved by #143.

[delphinus]

Ah, it's incomplete. Open it.

[delphinus]

We should introduce the only thread to access DB (save & load) by mpsc (multi producer and single consumer model).

[FelipeLema]

luv seems to provide either a new thread we can't communicate with or a thread pool we can communicate with

so it looks like it should be addressed with an API outside of luv... perhaps a two-way communication with an unix socket?

[delphinus]

I tried threading (not coroutines) to separate the logic to save/load DB, but it needs rewriting almost all logic because the current code depends on plenary.async module a lot.

Instead of that, I have written code to access DB more lazily in #180. That makes it postpone DB initialization until the first time :Telescope frecency is called. And load()/save() themselves are called in a separated coroutine not to affect the main loop.

I think it is enough for this issue. Closing.