ABCI: ResponseCheckTx and ResponseDeliverTx expects single-coin Fee

[AdityaSripal]

ABCI API Change Request:

Currently the ResponseCheckTx and ResponseDeliverTx structs both have a Fee attribute of type common.KI64Pair. This assumes that a single coin is used for the transaction fee which is an assumption not shared by the Cosmos-SDK as seen in StdFee and FeeCollectionKeeper in x/auth module which expects sdk.Coins for the FeeAmount.

Changing the Fee attribute in the ABCI response to []common.KI64Pair will allow the response and the corresponding sdk.Result to represent multicoin fees. This will make the ABCI more extensible and in-line with the cosmos SDK.

[AdityaSripal]

From discussion with @sunnya97:

Rather than include anything about FeeCoins into the response, we create a Priority uint that could be used in the future to order transactions from mempool

It's upto the application to calculate the Priority

[ebuchman]

Yeh! Closing for #1861