Rather than using a channel for notifictions, use a callback instead.
There are several issues that arise with async notifications via a
channel, so use a callback instead. The caller can always make the
notifications async by using channels in the provided callback if
needed.
After discussion with others and thinking about the notification channel
some more, we've decided to leave it up to the caller to quickly handle
notifications. While it is true that notification should be handled
quickly to not block the chain processing code unnecessarily, launching a
goroutine in chain means the notifications are no longer necessarily in
order. Also, if the caller is not properly handling the notifications,
the goroutines end up sicking around forever. By leaving it up to the
caller to quickly handle the notification or launch a goroutine as
necessary for the caller, it provides the flexibility to ensure proper
notification ordering as well as control over other things such as
how to handle backpressure.
Rather than sending the root of the an orphan chain when sending an orphan
notification, send the hash of the orphan block itself. The caller can
then call the GetOrphanRoot function with the hash to get the root of the
orphan chain as needed. This is being changed since it's cleaner and more
detministic sending the hash for the orphan block that was just processed
rather than sending a possibly different hash depending on whether there
is an orphan chain or not.
Since the notification channel is provided by the caller and it may or may
not be buffered, send notifications in their own goroutine so the chain
processing code does not have to wait around on the caller to process the
notification before continuing.
