Replace WatchEventProvider with chain::Notify

Replace WatchEventProvider with chain::Notify

WatchEventProvider served as a means for replacing ChainWatchInterface.
However, it requires users to explicitly fetch WatchEvents, even if not
interested in them. Replace WatchEventProvider by chain::Notify, which
is an optional member of ChainMonitor. If set, interesting transactions
and output spends are registered such that blocks containing them can be
retrieved from a chain source in an efficient manner.

This is useful when the chain source is not a full node. For Electrum,
it allows for pre-filtered blocks. For BIP157/158, it serves as a means
to match against compact filters.