`chain::Watch` should probably support BIP 157/158

For light clients using BIP 157/158 it'd be really nice to be able to use the `chain::Watch` API - its a chunk simpler and if you're syncing the header tree anyway it lines up much more directly with how you do so over P2P. To do this, I think basically the only change would be to add a `best_block_updated_without_matching_txn` method which just takes a header.