Membership upNumbers, make them actually work

[ktoso]

I don't like them to be honest... but they're easier than ad-hoc implementing a CAS thing right away.

See: https://github.com/apple/swift-distributed-actors/pull/376/files#diff-85762b080a33dde0f48096002d587956R40

Up numbers are assigned by the leader when doing the -> up move. They are an order of "age" the oldest member has the lowest up number. Thanks to this we know which node is the "oldest" and can get on that node being "less likely to get down" rather than some burst-y new node added during a spike.

I'd love to not have the numbers but implement a CAS / bully like singleton setting...