Add AIMultiGrid::LostAndFound

Made creating and cleanup of lost+found directory thread safe.
Add AIMultiGrid::LockedBackEnd::readAndCacheAllUploadedAssets

Split off this code into a separate function.
Add AIMultiGrid::BackEnd::rename_gridnick

and LockedBackEnd::rename_gridnick of course.

This might be needed in the future if some grid changes
their gridnick. Still have to add code to detect when
this might have happened and UI stuff to ask the user
if they want to do the rename etc.