Store thumbnails in database

[ghost]

Is your feature request related to a problem? Please describe.
filebrowser writes thumbnail images as .jpg files on disk. It may become slow to load and hard to move when the folder gets very large.

Describe the solution you'd like
Store thumbnails in a database, like lmdb. Purging the cache will be instantaneous.

[o1egl]

I think with that solution images fetching speed will drop significantly

[ghost]

@o1egl

I haven't compared them myself, but it seems storing with LMDB will increase the fetching speed, and there are other benefits.
simon987/sist2#127 (comment)