Qt Online Installer metadata cache

Idea proposal:

Support local machine cache for metadata to speed up consequent launches of the online installer.

For initial implementation:

1. installer stores the temporary local repository structures ("/tmp/remoterepo-XXXXXX" directories) in a defined cache location
2. checksums of new downloaded Updates.xml files are compared to checksums in cache
3. matching cache entries need to be checked for validity (i.e. meta.7z:s not deleted and their checksum matches)
4. download only metadata for missing repositories (Updates.xml sha not in cache) or broken metadata archives

• metadata archives could probably be uncompressed only once, but we would need to calculate a new checksum for the uncompressed content
• could we use a similar cache for the data archives?

For next steps:

1. releasing to incrementally update metadata checksum table per repository
2. this checksum table is updated to Qt back-end (steps 1-2 automated)
3. Qt Online Installer: after login, the Qt backend returns the checksum table
4. Those repositories that are located in the local cache do not need to be updated from online
5. When downloading new metadata from online these are stored into local IFW cache
6. Tag repositories/components which do not need script evaluation during init, evaluate only during the install phase
   1. tagging made by packaging
7. split control scripts in two:
   1. init phase
   2. install phase