Deprecate `tun.routes` in favor of `tun.paths`

[jasikpark]

Currently, tun.routes is a bit misleading as a config setting: it allows you to set custom MTU overrides for different nebula overlay IPs, because they might be on a local LAN with you, for example with two machines in the same AWS subnet, or two machines on the same office subnet.

We should deprecate this and move to tun.paths, which allows you to set certain subnets or IP addresses that have custom MTU settings so that you can say "the AWS local LAN IPs are allowed to have large windows" instead of "the AWS machines can have large windows", and then nebula is connecting via the public internet rather than LAN b/c it's faster, and you get TCP fragmentation

[noseshimself]

That one just bit me.