In this commit, we extend the NeutrinoNotifier 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. A concurrent
queue for relevant transactions has been added to proxy notifications
from the underlying rescan to the txNotifier. This is needed for
scripts, as we cannot perform a historical rescan for scripts through
`GetUtxo`, like we do with outpoints.
For scripts that are unspent, a filter update is sent to the underlying
rescan to ensure that we match and dispatch on the script when
processing new blocks.
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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