Rejected transaction patch when FFI `2.0.1` or `2.1.0` is released

[rustaceanrob]

With apply_evicted_txs added to BDK wallet, we can address one part of #319. When receiving a warning that a transaction was rejected for a too-low fee, we can remove it from pending transactions with apply_evicted_txs. This is assuming that apply_unconfirmed_txs has already been called. There is a nuance of using the peer-to-peer network for broadcasting, as all transactions are identified with Wtxid. To implement apply_evicted_txs:

• A map from Wtxid-> Txid has to be held in memory when a transaction is sent
• If a warning notifies us that a transaction has been reject, it must be serialized into a Wtxid, and the corresponding Txid will have to be found in the map
• Remove that Txid with apply_evicted_txs

[reez]

Thanks for adding this 👍