Investigate electrs UTXO lookup

[gregdhill]

There is a problem with the current light client implementation (see this comment) that we may attempt to fund a transaction with a stale UTXO (reported by Electrs) if there is a parallel payment in the mempool. Firstly, we should investigate and note when exactly UTXOs are marked as spent by Electrs and then implement one of the following:

1. Hold the transaction creation mutex until the block has been included and Electrs marks this as spent
2. Cache UTXOs we are spending from and skip these when funding a new transaction