Generate msgtx from decoratedTx

This commit separates the btcwire.MsgTx from the decoratedTx, but the
MsgTx can be generated from the decoratedTx via the toMsgTx function.

Further, this makes it easier to test these two entities separately,
making test clearer and easier to understand.
Change OutputRequest.address to a btcutil.Address and add a pkScript field to it

This allows us to get rid of the net field we had in a few of our data types
and several functions/methods