mempool: reject TXs that exceed consensus covenant block limits

[pinheadmz]

Users have been able to broadcast huge batches with 1,000 OPENs in a single TX. That would make an entire block invalid, so there's no reason to relay such a TX. We could also decide to add a POLICY rule that restricts this even further (the recent wallet batching work limits to like 200)

[coveralls]

Coverage increased (+0.008%) to 68.205% when pulling 2f027e947342935a3ea60a48adeebfa21fbaf35a on pinheadmz:policy-limits into 455bf60f8198e13949dc7d35dad162c67a50b337 on handshake-org:master.