Ensure mempool is persisted across restarts

As part of the larger plan to fix timeout errors, we identified that it's important to make sure our federation of Bitcoin nodes are persisting the mempool across restarts, in order to reduce the chances that Bitcoinj is unable to broadcast transactions due to being unable to verify dependent transactions that are no longer available in our nodes' mempools after restarting.

Per https://bitcoincore.org/en/releases/0.15.0/, it appears that 0.14.0 introduced the `-persistmempool` flag, and that it is enabled by default:


If it is in fact enabled by default, and we've verified that our nodes are behaving this way, then this issue can be closed out without any further changes.

Otherwise, we should add an option to the shared `bitcoin.conf` file to ensure that this option does get enabled on all nodes. In fact, we might want to add this option explicitly to our shared `bitcoin.conf` even if it is enabled by default, simply to communicate clearly / document what settings matter in our nodes. This could prevent a situation where one of our @bisq-network/btcnode-operators thinks it's okay to disable mempool persistence on their own node because they didn't see anything explicit about it in the shared `bitcoin.conf` file.