Let `TxBuilder` avoid previously used UTXOs (UTXO locking)

[tnull]

Describe the enhancement
When sequentially creating multiple transactions, TxBuilder might currently reuse the same UTXOs, potentially exposing us to double-spend ourselves if we want to bring all of these generated transactions on-chain. It would be nice if the TxBuilder could track the used UTXOs internally, and allow to auto-unlock them after a set amount of time.

Use case
This is important for sequentially opening multiple Lightning channels, e.g., in LDK Node.

[notmandatory]

We had some offline discussions about how to approach this issue, recapping here:

Step 1: create example of how to do it outside of bdk such as by manually keeping a set of used but not broadcast utxos, and then feeding that set to the TxBuilder to exclude the UTXOs when creating new transactions.

Step 2: based on experience with 1, create API changes for the Wallet (or TxBuilder?) that will make it possible to automatically manage the set of used but not broadcast UTXOs.

[LLFourn]

I think we want to support this in bdk_chain and bdk::Wallet explicitly. See cancel_tx in v1.0.0-alpha.0 PR: https://github.com/bitcoindevkit/bdk/blob/7e8f849d4708efa6102d5c25a7647c2b47b90339/src/wallet/mod.rs#L1385

The idea is when a tx is cancelled in the future we can mark those utxos as no longer used and return them to the "utxo" pool.

See also the necessary feature in bdk_chain: LLFourn/bdk_core_staging#186

There are other cases where you want to reserve utxos that will have to be done via a different method (manual marking) which we can also provide.

[evanlinjin]

Currently, the proposed solution to this is (for BDK 1.0):

KeychainTracker should have mark_spent and unmark_spent that work in a similar way to mark_used and unmark_used in KeychainTxOutIndex. The reason we want this is to be able to "reserve" utxos so other threads cannot use them when we've decide to construct a transaction using them.

However, as we are replacing the KeychainTracker with IndexedTxGraph (as specified in bitcoindevkit/bdk#895), these methods should be added to IndexedTxGraph.

[LLFourn]

It's a little bit harder to say that IndexedTxGraph should have this method. Perhaps this should just exist in bdk::Wallet for now by storing a HashSet so you can reserve things.