MsgFee Enhancements for Low/Zero Fee transactions

[iramiller]

Summary

Allowing low or no cost msgs exposes the chain to a new set of risks that the traditional gas cost was intended to prevent. As part of adding support for this approach a few changes are required to ensure that msgs do not use excessive amounts of gas/blockspace when there is no cost.

Problem Definition

One of the common configurations expected for a private zone is to have a zero gas price since the network infrastructure will often be ran entirely by a single entity. In these configurations there is no need to collect and distribute gas fees to the rest of the infrastructure providers. As a related point there are some transactions that it makes sense to allow for free (such as in certain cases where IBC transactions maybe subsidized). These transactions should still be constrained to a reasonable amount of gas due to the hard limits on the amount of gas available in each block.

Proposal

Update the msgfee module to better reflect its intention of pricing certain msgs and controlling their use. Specifically add the ability to set a maximum gas allowed for each msg type in addition to a specifying a fee (or lack there of). Ensure that when a multiple msg transaction is submitted that it does not use more gas than is allowed for each transaction.

Note: This is a bit of a dangerous limit to put in place because it can prevent valid transactions from processing if the gas limits are too low or the chain cost rise for some unexpected reason. It might be useful to track a running average of gas used per msg type along with minimum/maximum values for successful transactions that include the msg type.

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