Expect callers to hold read locks before channel_monitor_updated

Our existing lockorder tests assume that a read lock on a thread
that is already holding the same read lock is totally fine. This
isn't at all true. The `std` `RwLock` behavior is
platform-dependent - on most platforms readers can starve writers
as readers will never block for a pending writer. However, on
platforms where this is not the case, one thread trying to take a
write lock may deadlock with another thread that both already has,
and is attempting to take again, a read lock.

Worse, our in-tree `FairRwLock` exhibits this behavior explicitly
on all platforms to avoid the starvation issue.

Sadly, a user ended up hitting this deadlock in production in the
form of a call to `get_and_clear_pending_msg_events` which holds
the `ChannelManager::total_consistency_lock` before calling
`process_pending_monitor_events` and eventually
`channel_monitor_updated`, which tries to take the same read lock
again.

Luckily, the fix is trivial, simply remove the redundand read lock
in `channel_monitor_updated`.

Fixes #2000

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -4101,7 +4101,7 @@ where
 	}
 
 	fn channel_monitor_updated(&self, funding_txo: &OutPoint, highest_applied_update_id: u64, counterparty_node_id: Option<&PublicKey>) {
-		let _persistence_guard = PersistenceNotifierGuard::notify_on_drop(&self.total_consistency_lock, &self.persistence_notifier);
+		debug_assert!(self.total_consistency_lock.try_write().is_err()); // Caller holds read lock
 
 		let htlc_forwards;
 		let (mut pending_failures, finalized_claims, counterparty_node_id) = {
@@ -5068,6 +5068,8 @@ where
 
 	/// Process pending events from the `chain::Watch`, returning whether any events were processed.
 	fn process_pending_monitor_events(&self) -> bool {
+		debug_assert!(self.total_consistency_lock.try_write().is_err()); // Caller holds read lock
+
 		let mut failed_channels = Vec::new();
 		let mut pending_monitor_events = self.chain_monitor.release_pending_monitor_events();
 		let has_pending_monitor_events = !pending_monitor_events.is_empty();