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graph: refactor Builder network message handling

The exposed AddNode, AddEdge and UpdateEdge methods of the Builder are
currently synchronous since even though they pass messages to the
network handler which spins off the handling in a goroutine, the public
methods still wait for a response from the handling before returning.
The only part that is actually done asynchronously is the topology
notifications.

We previously tried to simplify things in [this
commit](https://github.com/lightningnetwork/lnd/pull/9476/commits/d757b3bcf)
but we soon realised that there was a reason for sending the messages to
the central/synchronous network handler first: it was to ensure
consistency for topology clients: ie, the ordering between when there is
a new topology client or if it is cancelled needs to be consistent and
handled synchronously with new network updates. So for example, if a new
update comes in right after a topology client cancels its subscription,
then it should _not_ be notified. Similariy for new subscriptions. So
this commit was reverted soon after.

We can, however, still simplify things as is done in this commit by
noting that _only topology subscriptions and notifications_ need to be
handled separately. The actual network updates do not need to. So that
is what is done here.

This refactor will make moving the topology subscription logic to a new
subsystem later on much easier.