make cluster UUIDs distinguishable from any other repository UUID

make cluster UUIDs distinguishable from any other repository UUID

A cluster UUID is a version 8 UUID, with first octets 'a' and 'c'.
The rest of the content will be random.

This avoids a class of attack where the UUID of a repository is used as
the UUID of a cluster, which will prevent git-annex from updating
location logs for that repository. I don't know why someone would want
to do that, but let's prevent it.

Also, isClusterUUID make it easy to filter out cluster UUIDs when
writing the location logs.