Add reasonable rate limits to grinbox

[lehnberg]

As a team paying for the hosting of grinbox servers, we need grinbox to reasonably rate limit usage from unique IPs, so that companies do not take the piss and use us as infrastructure they ought to be hosting themselves.

[lehnberg]

Adding this will require us to store some data, for some time on users... I think. Which is not great. Hmm... 😐

https://github.com/vault713/wallet713#privacy-considerations

[theymos]

Require a proof-of-work which will take a few seconds to complete on average computers.