Define txpool ownership

[notlesh]

Designing and running a txpool is a lot of work. It would obviously be ideal if a container-chain project didn't have to run any infra for its txpool, but pushing all of this infra on the tanssi collators comes with a lot of conflicts-of-interest.

Some things to consider:

• Tanssi collators should have incentives to include container-chain txns
• The performance of one container chain's txpool should not [drastically] impact others
• The overall protocol should prevent intentional or unintentional abuse, which can come in different flavors:
  • gossip spam
  • excessive validate() overhead
  • frontrunning and other bad incentives

Even if container chains don't need to run their own txpool infra, it's still extremely important that their txn design does not lead to performance problems.

TODO:

• Modify purchase_credits() to buy up to the credits specified to tolerate failure/abuse better