This commit adds a new method to the ChainNotifier interface which
subscribes the caller to a continuous stream of notifications generated
by new blocks added to the tip of the Bitcoin main chain.
Concurrently, this method is intended to be used in order to obtain the
necessary block height information to properly handle the timeout
period on any pending HTLCs. A continuos stream, rather than a one-off
notification is chosen in order to discourage a goroutine-per-HTLC
model which would be rather wasteful.
* Turns out the NotificationServer on btcwallet doesn’t sever full
blocks, nor notify for arbitrary transactions. Instead, we now create a
new client specifically for BtcdNotifier.
* Final implementation is simpler, less dependent on newer btcwallet
features in flux. Additionally, this decouples the chain notifications
from the wallet. Enabling reliance on btcd for notifications, in
conjunction with an independent wallet that satisfies the to-be-drafted
Wallet interface.
* Rather than rely on the legacy notification interface which may be
deprecated in the near future, we’ll now switch to using the
NotificationSever.
* Negative confirmation notifications due to re-orgs are still
unimplemented however.
* Re-orgs are still unhanded.
* RegisterSpendNtfn should perhaps also register directly with the rpc
client instead of pushing the responsibility to the caller.
* Currently depends on the wallet, but that will change in the near
future due to new additions to btcwallet currently under review.
* Re-orgs aren’t intelligently handled yet. To be done along with tests
in a later commit.
