In this commit, we extend the BtcdNotifier to support registering
scripts for spends notifications. Once the script has been detected as
spent within the chain, a spend notification will be dispatched through
the Spend channel of the SpendEvent returned upon registration.
For scripts that have been spent in the past, the rescan logic has been
modified to match on the script rather than the outpoint. This is done
by encoding the script as an address.
For scripts that are unspent, a request to the backend will be sent to
alert the BtcdNotifier of when the script was spent by a transaction. To
make this request we encode the script as an address, as this is what
the backend uses to detect the spend. The transaction will then be
proxied through the txUpdates concurrent queue, which will hand it off
to the underlying txNotifier and dispatch spend notifications to the
relevant clients.
Along the way, we also address an issue where we'd miss detecting that
an outpoint/script has been spent in the future due to not receiving a
historical dispatch request from the underlying txNotifier. To fix this,
we ensure that we always request the backend to notify us of the spend
once it detects it at tip, regardless of whether a historical rescan was
detected or not.
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