Replace use of ChainWatchInterface with WatchEvent

Replace use of ChainWatchInterface with WatchEvent

SimpleManyChannelMonitor is parameterized by ChainWatchInterface to
signal what transactions and outputs to watch for on chain. The
interface has grown to cover chain access (via get_chain_utxo) and block
block filtering (via filter_block and reentered), which has added
complexity for implementations and user (see ChainWatchInterfaceUtil).

Pull the watch functionality out as a first step to eliminating
ChainWatchInterface entirely.