Use generics for CIDRTrees to avoid casting issues

[nbrownus]

Had a report in Slack of an issue with use_system_route_table. Figured it would be best to make the CIDRTrees safer to use

panic: interface conversion: interface {} is *interface {}, not iputil.VpnIp

goroutine 83 [running, locked to thread]:
github.com/slackhq/nebula/overlay.(*tun).RouteFor(0xc000000240?, 0x0?)
        github.com/slackhq/nebula/overlay/tun_linux.go:159 +0x85
github.com/slackhq/nebula.(*Interface).getOrHandshake(0xc000202000, 0xc?, 0xc?)
        github.com/slackhq/nebula/inside.go:113 +0x45
github.com/slackhq/nebula.(*Interface).consumeInsidePacket(0xc000202000, {0xc00045d400, 0x3c, 0x2329}, 0xc0000133b0, {0xc0000133c0, 0xc, 0xc}, {0xc00045f900, 0x2329, ...}, ...)
        github.com/slackhq/nebula/inside.go:47 +0x22a
github.com/slackhq/nebula.(*Interface).listenIn(0xc000202000, {0x7f7e7ddbb838, 0xc0000acb80}, 0x0?)
        github.com/slackhq/nebula/interface.go:290 +0x14b
created by github.com/slackhq/nebula.(*Interface).run in goroutine 1
        github.com/slackhq/nebula/interface.go:248 +0x99