Rebroadcast/bumping of on-chain wallet transactions

[tnull]

Post-#358 we should look into better tracking of on-chain transactions (also do #67), and implement rebroadcasting / bumping for them.

[tnull]

When we do this, we should also see if we can improve our failure handling around the on-chain wallet, e.g., undoing/not persisting any changes that arise from a failed TxBuilder flow, see #358 (comment) and the first example here: tnull@8ed0783