Flush receipt with the correct state (#546)

## Describe your changes and provide context
`stateToCommit` could be either `processProposalState` or
`deliverState`, so we should flush to the correct one. This might not be
causing issue yet though, since before receipt flush, we'd first write
`stateToCommit` to the parent state, so that `deliverState` would fall
through to parent which is already updated by `stateToCommit`

## Testing performed to validate your change
existing test

baseapp/abci.go

@@ -372,7 +372,7 @@ func (app *BaseApp) Commit(ctx context.Context) (res *abci.ResponseCommit, err e
 	retainHeight := app.GetBlockRetentionHeight(header.Height)
 
 	if app.preCommitHandler != nil {
-		if err := app.preCommitHandler(app.deliverState.ctx); err != nil {
+		if err := app.preCommitHandler(app.stateToCommit.ctx); err != nil {
 			panic(fmt.Errorf("error when executing commit handler: %s", err))
 		}
 	}