Cleanup: avoid multiple `!` in MembershipDSL

[ktoso]

Today we do:

return (.uniqueNode(nodeById(id: v.first!)), VersionVector.Version(v.dropFirst().first!)!)

try to avoid all those !!!

See https://github.com/apple/swift-distributed-actors/pull/438/files#diff-5cf4273f0ced27d0d56e61de001ccbf5R85