Fix persistence-required futures always completing instantly

[TheBlueMatt]

After the first persistence-required Future wakeup, we'll always complete additional futures instantly as we don't clear the "need wake" bit. Instead, we need to just assume that if a future was generated (and not immediately drop'd) that its sufficient to notify the user.

[codecov-commenter]

Codecov Report

Base: 90.74% // Head: 92.01% // Increases project coverage by +1.27% 🎉

Coverage data is based on head (2004451) compared to base (3b2f694).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head 2004451 differs from pull request most recent head f382f56. Consider uploading reports for the commit f382f56 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1845      +/-   ##
==========================================
+ Coverage   90.74%   92.01%   +1.27%     
==========================================
  Files          87       89       +2     
  Lines       47364    61307   +13943     
  Branches    47364    61307   +13943     
==========================================
+ Hits        42980    56412   +13432     
- Misses       4384     4895     +511     

Impacted Files	Coverage Δ
lightning/src/ln/channelmanager.rs	88.36% <100.00%> (+3.17%)	⬆️
lightning/src/util/wakers.rs	95.93% <100.00%> (+4.37%)	⬆️
lightning/src/chain/mod.rs	66.66% <0.00%> (-1.52%)	⬇️
lightning/src/util/events.rs	37.89% <0.00%> (-0.51%)	⬇️
lightning-background-processor/src/lib.rs	96.01% <0.00%> (-0.23%)	⬇️
lightning/src/lib.rs	100.00% <0.00%> (ø)
lightning/src/ln/reorg_tests.rs	100.00% <0.00%> (ø)
lightning/src/offers/offer.rs	94.56% <0.00%> (ø)
lightning/src/util/string.rs	100.00% <0.00%> (ø)
lightning/src/ln/features.rs	99.77% <0.00%> (+0.10%)	⬆️
... and 31 more

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[TheBlueMatt]

Fixed the doc links, note that we can't actually link await_persistable_update_timeout from await_persistable_update as the former is std-only and the latter is always available.

[TheBlueMatt]

Squashed, the fixup commit after @wpaulino's review was:

$ git diff-tree -U1 57855ff9c0b62d4a05b77bb512a3694e2dcaec8b f382f56c
diff --git a/lightning/src/util/wakers.rs b/lightning/src/util/wakers.rs
index 92fdd0165..655fc9cf7 100644
--- a/lightning/src/util/wakers.rs
+++ b/lightning/src/util/wakers.rs
@@ -95,10 +95,12 @@ impl Notifier {
 		}
-		if !future_probably_generated_calls {
-			// If no future made any callbacks and has been drop'd (i.e. the state has only the one
-			// reference we hold), assume the user was not notified and set the
-			// notification-required flag. This will cause the `wait` functions above to return.
-			lock.0 = true;
+		if future_probably_generated_calls {
+			// If a future made some callbacks or has not yet been drop'd (i.e. the state has more
+			// than the one reference we hold), assume the user was notified and skip setting the
+			// notification-required flag. This will not cause the `wait` functions above to return
+			// and avoid any future `Future`s starting in a completed state.
+			return;
 		}
+		lock.0 = true;
 		mem::drop(lock);
-		if !future_probably_generated_calls { self.condvar.notify_all(); }
+		self.condvar.notify_all();
 	}