A height hint not being set would cause lnd to scan for the
confirmation/spend of a txid/outpoint/address from genesis.
The number of confirmations not being set within a confirmation request
would cause the internal TxNotifier to deadlock when dispatching
updates.
In this commit, we refactor the HeightHintCache and its underlying
interfaces to be able to manipulate hints for ConfRequests and
SpendRequests. By doing so, we'll be able to manipulate hints for
scripts if the request includes either a zero hash or a zero outpoint.
In this commit, we modify the TxNotifier's ConnectTip method to no
longer dispatch notifications to any clients who had a request fulfilled
within the height connected. Instead, it will queue the notifications
for dispatch and we add a new method NotifyHeight, which will actually
dispatch them. We do this to allow the users of the TxNotifier to be
more flexible when dispatching notifications.
