Ability to turn off full thumb-blog rescan after new version update

[apoapostolov]

A new config.toml option to turn off full thumb-blog scan when new version is used.
The option should be off by default (allowing full scan) but user can turn it on.

This will save a lot of trouble after minor upgrades where just because a minor fix is done, a full redownload is needed of hundreds to thousands of archived blogs. In my case I've noticed a lot more old content is being re-downloaded which may or may not be old hardlinked content, but either way it's not like it is needed, and should have already been downloaded.

[Liru]

I think I'll just disable the post-update scan completely.

[apoapostolov]

Thanks, that is really appreciated.