Metadata download improvements

Business justification

Qt users would benefit a nicer installation experience. One place for improvement is to make meta-data download faster and make the download happen less frequently. This would especially have a positive effect on the Qt UX for evaluators. 

Areas of improvements 

The ideas and suggestions to improve the performance are collected here. 

Reduce the number of metadata downloads:

• This step happens every time I open the installer, which is fine if I haven't used it for hours, but if I used it 10 seconds ago, it shouldn't have to take 30 seconds to download meta information. The time the download takes seem to be the same regardless of how recently it occurred, which suggests that it's not very efficient. If the packages are identified by some kind of hash, then this process should be almost instant.
• Stop creating and downloading empty meta.7z. Currently almost half (1334/3081 on linux) of the meta.7z's are essentially empty, Downloading and extracting those just wastes time and resources.
• Combine all the individual component meta.7z's in a repository, or more precisely their content, into one meta.7z per repository. Since all the meta.7z's are downloaded anyways, we could have all of the content in a single archive, which would save on TCP connections.