x/txfees: Make an ante-handler filter for recognizing High gas txs, and having a min gas price for them

[ValarDragon]

Similar to #723 , we should make a mempool filter for high gas fee txs to have non-zero fee requirements. (Configurable by every full node operator)

E.g. if gas fee > $THRESHOLD, require minimum-gas-price to be at least x.

Would require two params in the app.toml config threshold and x. Though maybe its fine to just hardcode the threshold?

I think its probably worthwhile for governance to consider requiring this in the state machine. (Though that could also be premature with cosmwasm, unclear how cosmwasm contracts will perform gas-wise)

[ValarDragon]

I suggest we hard code first $THRESHOLD=1 million gas, and get a config param for MINIMUM_GAS_PRICE.

We can probably implement this first with MINIMUM_GAS_PRICE = .01uosmo as default, and then work on getting it into the config.

[hleb-albau]

took it