Exclude pending UTXOs from coin selection API

[Voxelot]

To ensure optimal concurrency, when performing coin selection (coins_to_spend) we should filter out any unspent but pending utxos from the txpool. This way users can submit multiple transactions at once without conflicting inputs.

We'll also need the ability to provide a list of utxo ids to exclude from the search. This would allow the wallet to track pending utxos which might not have been synced to the local txpool yet.