The ABCI protobuf uses RequestXxx types for all methods.

The ABCI interface we use in Go uses RequestXxx for all methods except CheckTx and DeliverTx because they were always just tx []byte.

However, issues like #2127 suggest we may need to provide more than just tx []byte. So we should start using RequestXxx for CheckTx and DeliverTx in the Go interface.

This is non-breaking on ABCI itself, just on the Go API. Any code that imports the abci package would be affected.