Add option to adjust the max parallel remote thumbnail fetch for SSH

[CanglanXYA]

If you use SSH related remote file share service , the maximum session will be restricted by remote SSH, and the app will stop fetch thumbnails and crash after two many invalid requests.

Allow user to fetch one thumbnail at once will prevent the app ANR and crashing, or just let user decide how many concurrencies they need for fetch thumbnails according to their SSH configure file.