Exclude high priority economic cosmos tx from gas based backpressure

[mhofman]

What is the Problem Being Solved?

We need high-priority economic messages from things like oracles to reach swingset regardless of the load conditions of our chain.
We currently do not have a variable gas design based on the current load, but we did consider it at some point (possibly discussed in this long issue: #3103)

If we ever start requesting higher gas for messages when the kernel queues are full, we should exempt high priority senders from the policy.

Description of the Design

TBD

Security Considerations

Make sure that this exemption cannot be abused

Test Plan

TBD

[warner]

cc @gibson042