Make tx_abort a variant of InteractiveTxMessageSend and refactor

[dunxen]

Shouldn't hold up this PR, but is there any reason why we don't simply add a TxAbort variant to InteractiveTxMessageSend? Then we wouldn't need either InteractiveTxMessageSendResult or HandleTxCompleteResult, IIUC, which would remove two layers of structs.

Otherwise, we go through the following conversions just to end up with a MessageSendEvent:

• Result<InteractiveTxMessageSend, AbortReason>
• InteractiveTxMessageSendResult(Result<InteractiveTxMessageSend, msgs::TxAbort>)
• MessageSendEvent

The Results don't seem to buy us anything more than the extra variant would as we aren't doing anything special for the Err case. Then it would simply be:

• InteractiveTxMessageSend
• MessageSendEvent

And AFAICT, InteractiveTxConstructor has the channel_id, so there's no need to grab it from the ChannelContext. Or will it change in some case?

Originally posted by @jkczyz in #3137 (comment)