When we don't care about the payload value and are just checking whether
a set contains an IP/prefix, we can use `bart.Lite` for the same lookup
times but a lower memory footprint.
Fixes#19075
Change-Id: Ia709e8b718666cc61ea56eac1066467ae0b6e86c
Signed-off-by: Alex Chan <alexc@tailscale.com>