This change modifies six structs that were keeping
track of anchors features with an `opt_anchors` field,
as well as another field keeping track of nonzero-fee-
anchor-support.
Certain users may not care how their UTXOs are selected, or their wallet
may not expose enough controls to fully implement the
`CoinSelectionSource` trait. As an alternative, we introduce another
trait `WalletSource` they could opt to implement instead, which is much
simpler as it just returns the set of confirmed UTXOs that may be used.
This trait implementation is then consumed into a wrapper `Wallet` which
implements the `CoinSelectionSource` trait using a "smallest
above-dust-after-spend first" coin selection algorithm.
This allows users to bump their commitments and HTLC transactions
without having to worry about all the little details to do so. Instead,
we'll just require that they implement the `CoinSelectionSource` trait
over their wallet/UTXO source, granting the event handler permission to
spend confirmed UTXOs for the transactions it'll produce.
While the event handler should in most cases produce valid transactions,
assuming the provided confirmed UTXOs are valid, it may not produce
relayable transactions due to not satisfying certain Replace-By-Fee
(RBF) mempool policy requirements. Some of these require that the
replacement transactions have a higher feerate and absolute fee than the
conflicting transactions it aims to replace. To make sure we adhere to
these requirements, we'd have to persist some state for all transactions
the event handler has produced, greatly increasing its complexity. While
we may consider implementing so in the future, we choose to go with a
simple initial version that relies on the OnchainTxHandler's bumping
frequency. For each new bumping attempt, the OnchainTxHandler proposes a
25% feerate increase to ensure transactions can propagate under
constrained mempool circumstances.
While users could easily figure it out based on the set of HTLC
descriptors included within, we already track it within the
`OnchainTxHandler`, so we might as well expose it to users as a
nice-to-have. It's also yet another thing they must get right to ensure
their HTLC transaction broadcasts are valid.
