Avoid transaction spam

Today there is absolutely no constraint about how many transactions from a single user can be included in a single block. I think this is a particularly big security flaw.

We should at least put a limit on the number of chained transactions that can be included in a single block. With such a measure, at least attackers would have their attack limited by block computation.

Note: this fix should trigger itself on a particular day, planned few time after when we expect the 1.6 to be released.

A good improvement would also include larger fixes already described in this thread: https://forum.duniter.org/t/sans-frais-de-transaction-comment-resister-aux-attaques/3846