Many places that need to work with node/peer capabilities end up with a
something-View and need to either reimplement the helper code or make an
expensive copy. We have the machinery to easily handle this now.
Updates #cleanup
Change-Id: Ic3f55be329f0fc6c178de26b34359d0e8c6ca5fc
Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>