perf: replace the ipv6 map by an array of struct

[Jorropo]

While starting ipfs this package used to take ~20ms while initing:

init github.com/libp2p/go-libp2p-asn-util @21 ms, 20 ms clock, 2623128 bytes, 892 allocs

That is because the map literal is actually creating a new map and adding entries one by one, which for such big map takes a while.
This PR moves this to a separate goroutine and use a sync.Once to synchronise accesses. Almost completely removing it:

init github.com/libp2p/go-libp2p-asn-util @8.7 ms, 0.002 ms clock, 128 bytes, 3 allocs

Never is this map actually used as a map, the only consumer just iterate over it.
So we can use an array of structs containing k and v instead, this pr make the init 0.005ms.

[Jorropo]

@willscott let's not merge this. I have red the code and this change is overkill.

Never is this map actually used as a map, the only consumer just iterate over it.

So we can use an array of structs containing k and v instead.

[Jorropo]

@willscott Update it's now slower 0.005ms instead of 0.002ms however it's 0.005ms of CPU time instead of 0.002ms of wall time and 20ms of CPU time.