Add possibility to promote updates based on checksum

A change in content checksum indicates that the content has changed. This should indicate the need to promote an update. Version number update is promoted by default, but it should be possible also to promote the update based on checksum.

Why this suggestion:

Currently roll-backs are not possible.
Consider example where 3.x is updated to 4.x but a critical issue is found and the update has been in production so that users have downloaded it already. It would not be possible to roll-back to 3.x or even to 3.x+1 to roll-back the situation to where it was ok. Versioning 3.x to "4.x" (and to promote that as an update fix) is not an option.

If multiple repositories exist with the same component with different checksums then the one with higher version number should be preferred. 

Now that 4.x is getting closer there is a place for potential API break.