spv validation using chain work instead of block height

[mariocynicys]

In SPV validation for lightning, we query electurms to find the most synced one and use the block height of it as our target block height. We determine the "most synced" by having the greatest block height.

We might have a bad electrum on a forked chain that has larger block count but less overall work, by consensus, such chain isn't the correct chain.

https://github.com/KomodoPlatform/komodo-defi-framework/blob/c1175ceddb2db4c8e7948b6c764bfd53aea1f8c0/mm2src/coins/utxo/rpc_clients/electrum_rpc/client.rs#L813-L816

We should be able to rule out such electrums and remove them from our connection list.

Relevant convo: #2666 (comment)

[shamardy]

As discussed, we validate that the blocks meet their target difficulty, but we don't always use the chain that has the most cumulative work. So there are edge cases where we would follow a different chain but this chain in time will not meet the target difficulty as it's not economically viable (for BTC at least).