Refactor to stop using the txbuilder more than once

[evan-forbes]

When creating MsgWirePayForMessages, we currently use the same txbuilder more than once. https://github.com/celestiaorg/lazyledger-app/blob/3192328a0952e6542395263fedc57b66c265ff79/x/payment/types/wirepayformessage.go#L47-L51

This will result in the generation of transactions that contain multiple messages when we only want one. We also need to modify this test to ensure that this works with multiple square sizes

PS: I think that merging such a large PR in #129 contributed to sneaking this bug in. Perhaps if I would have broken that PR up or if we would have had more time, then we would have caught this before hand.