addrAssignments is a table of addrs with lookup indices, representing
the assignments of magic+destination+transit IP addresses the client has
made dut to the domain being routed because of an app
.
byConnKey is a map of node public key to prefixes of transit IPs, so it
is associated with, but not that data itself, and can be its own thing.
Updates tailscale/corp#39975
Signed-off-by: Fran Bull <fran@tailscale.com>