WIP: Siadir cache FileInfos for faster lookup

This MR adds caching for FileInfos to the .siadir files. That way, instead of having to load every single file of a dir and compute the FileInfo, we just get the info directly from the .siadir file without having to compute the info again.

The downside is that we need to be careful to update the metadata every time one of the fields of the FileInfo of a SiaFile might change. Another downside is that we no longer get real time information on the health of the files but instead the same health information the repair loop has as the repair loop will be responsible for updating the cached FileInfos every time it checks the health of a file.