Support new version number scheme

After discussions, we've decided to change the image version numbering scheme to be sequential from 1. There are no additional semantics associated with version numbers. The only constraint is that version numbers must fit in 16 bits.

The client must be updated to a new hashing algorithm for Image objects to look at the version number (in full updates) or base+version (in deltas). If the version numbers are < 20000000 (could just be 20130000 but some existing tests use earlier years), then just use the version number directly (after verifying they are <= 16 bits wide). For version numbers >= 20000000 we continue to use the old hashing algorithm.

Nothing about candidate path calculation changes. E.g. 20130835 is still > 1 so once the new version numbers are available on the server, you will have to force update via `system-image-cli -b 0 --filter=full`.

Since this is very high priority, I will add a few tests of the new version numbering scheme, but I won't modify any existing tests that use the old version number scheme. Best effort will be made to test this against a live channel with new version numbers, but we accept that some bugs in the client may still after release requiring manual intervention. Once we retire the old version numbers, a tech-debt bug will be opened to fix all the old tests.