Add PersistenceNotifier to ChannelManager

This will allow the ChannelManager to signal when it has new
updates to persist, and adds a way for ChannelManager persisters
to be notified when they should re-persist the ChannelManager
to disk/backups.

Feature-gate the wait_timeout function because the core
lightning crate shouldn't depend on wallclock time unless
users opt into it.

Author: valentinewallace

lightning/Cargo.toml

@@ -11,6 +11,7 @@ Still missing tons of error-handling. See GitHub issues for suggested projects i
 """
 
 [features]
+allow_wallclock_use = []
 fuzztarget = ["bitcoin/fuzztarget", "regex"]
 # Internal test utilities exposed to other repo crates
 _test_utils = ["hex", "regex"]
@@ -37,3 +38,6 @@ features = ["bitcoinconsensus"]
 [dev-dependencies]
 hex = "0.3"
 regex = "0.1.80"
+
+[package.metadata.docs.rs]
+features = ["allow_wallclock_use"] # When https://github.com/rust-lang/rust/issues/43781 complies with our MSVR, we can add nice banners in the docs for the methods behind this feature-gate.