Don't double-spend ourselves on sequential channel opens

Unfortunately `bdk::TxBuilder` will reuse already used UTXOs, if no `sync()` call happens in-between.

We might therefore need to track UTXOs used in funding transactions and explicitly forbid reuse via `TxBuilder::unspendable`. Moreover we'll need to track current sync height and un-lock the UTXOs if they haven't been spent for some time to allow reuse.