Fixes #5553
Queued on #6327

The code in its current form is very crude and in no way ready to be merged.
This PR achieves a 8x speedup compared to 2.3.0~rc1 on subsequent empty updates after initialization of the new values, going from 3.5s to an almost instantaneous 0.4s.

TODO:

• make etag and last_modified their own record to avoid having to carry both around
• write the repos-config file when there is no diff but etag&last_modified differ
• bump the opam_version of repos-config to 2.4 (new syntax is not ignored by previous versions of opam)
• add some dedicated tests
• make sure the cache is invalidated