CircleCI : specialized caches

[tlaurion]

This changes the way CircleCI handles cache and limit the possibility of cache corruptions and removes the CircleCI owner bounding that was there previously.

Now:

• If modules/* and patches/* have the same hashes, that cache is restored. It saves the new cache if the hashes are different. This cache is usefull for most changes happening in Heads, which are security policy related and linked to scripts changes. If modules or hashes changes and no cache exists, if will restore the next cache.
• If musl-cross module and applied patches have the same hashes of an existing cache, that cache is restored. It is saved once for a single hash which means that the module version (commit) and applied patches are the same. This permits to economize one hour of build time for each build and rarely ever change.

[tlaurion]

Fixes #769