Rate limiting

[julien51]

We may want to rate limit to make sure people are not able to abuse since we don't actually "link" Twitter accounts.
The easiest way to do that which does not require us to store special state is to look at the subgraph for any user for whom we are about to create a lock and prevent them from doing it if they have create another lock less than say 1 hour ago?
In the subgraph, we keep the time (at least block number) for each lock created.