PendingHTLCsForwardable Events generating during `process_pending_htlc_forwards` are deduped out

If we get a call to `process_pending_htlc_forwards` while processing a `PendingHTLCsForwardable` event (ie the users' event handler method calls back into `ChannelManager`) we won't generate a new `PendingHTLCsForwardable` as the deduplication will kick in and see the existing in-process one in the queue. This at least can break for phantom payments.