Gossip txs based on priority

[evan-forbes]

Currently, while mempools like CAT and v1 have the ability to reap prioritized transactions from the mempool, they add transactions to be gossiped to a queue in a FIFO way. When under high load, this could result in not being able to get a high priority transaction included as it would take too long to gossip.

The idea proposed by @musalbas was to insert the transactions in some flavor of a sorted queue. I say some flavor as there were a few different proposals to avoid the overhead of having to always maintain a separate perfectly sorted queue.

One of the suggested solutions was to utilize the existing sorted queue inside the v1 and CAT mempools. Here we would keep track of which transactions are gossiped already and iterate through the queue of sorted un-gossiped transactions periodically or after some trigger.

Another of the suggestions was to add some conditions that must be met before any form of sorting only occurred, for example large high priority transactions or some threshold of very high priority.

[adlerjohn]

Aren't the mempools constantly sorted in order to be able to drop least-priority txs in the event the mempool fills up?

[musalbas]

The default max mempool size is something like 5000 transactions, so it can get quite large and exceed what can be gossiped quickly.