please make it async

[justrajdeep]

hi

Great plugin, I use it all the time, but it hangs for a while while it tries to save the session periodically. With vim8 async support can you please make this async?

Thanks

[mMontu]

It'd be great to have async support, not only for the delay while saving, but also to have some checkpoint along the day to prevent losing the session state in some unexpected events, like the machine/application crashing and power outages.