Limit the number of transactions and limit deployment priority in the mempool

[vicsn]

This PR may have become unneccesary due to https://github.com/ProvableHQ/snarkOS/pull/3089

Motivation

It is unfortunate that we can't take speculative execution fees from deployments at the moment, so as a temporary fix this should prevent DoS attacks.

Setting the priority_fee to 0 means that in theory, there could be a situation where deployments will never come to the front of the queue. But then we're in a scenario where we are bombarded with fee-paying executions, which means the community has enough fun with the existing deployments. Alternatively, we could create a separate deployments_queue where we can always pop some off.