graph: ensure topology subscriber handling and network msg handling is synchronous

[ellemouton]

We want to ensure that when a caller cancels a topology subscription, then any channel/node updates after that are not notified on the subscription. To ensure this, we make the Cancel function blocking.

In #9476, we removed announcement update handling (case update := <-b.networkUpdates:) from the main networkHander loop since the handling of any updates was immediately spun off into a goroutine anyways.

However, this did serve some purpose because it ensured that if we did the following:

1. cancel our subscription
2. immediately send an update

then case ntfnUpdate := <-b.ntfnClientUpdates: would be hit first ensuring that the <-b.networkUpdates case could not be triggered. Then by the time the update is handled, when it went to notify subscribers, the subscriber would have already been removed from the set of clients.

The same type of thing can happen if a new topology subscriber is registered and very soon thereafter an update arrives. In this case, a notification might be missed.

So this PR just reverts this change.

Fixes #9514

part of #9494

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[bhandras]

LGTM, thank your for the quick fix! 🥇

[yyforyongyu]

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