Limit amount of data published by Box

[michielbdejong]

Right now, a Box can call:

curl -X POST https://knilxof.org:4243/register -d "{\"local_ip\":\"some...very...long...string\",\"tunnel_url\":\"b\"}"

and we don't limit how long that string is. We should harden this a bit; this is a follow-up to: #11 (comment)

When limiting the length of the string sent in one call, we probably also want to limit the number of calls per second.

We can do this at the same time where we drop the formatting restrictions on the string sent (these restrictions don't really contribute anything apart from complicating our server upgrade path), see #11 (comment)