`OffersMessageHandler` impl for `ChannelManager` accesses time

[TheBlueMatt]

We have users who can't access time due to being in WASM, and while we'd like to move them to no-std, we can't do that until we get to rust-bitcoin 0.32. Thus, for our next release, we either need to upgrade to rust-bitcoin 0.32 or we need to drop the time accesses in the OffersMessageHandler impl.

[jkczyz]

Note that timer_tick_occurred is also affected for removing stale payments (i.e., stuck AwaitingInvoice).

rust-lightning/lightning/src/ln/channelmanager.rs

Lines 5711 to 5722 in 2701bc5

	#[cfg(feature = "std")]
	let duration_since_epoch = std::time::SystemTime::now()
	.duration_since(std::time::SystemTime::UNIX_EPOCH)
	.expect("SystemTime::now() should come after SystemTime::UNIX_EPOCH");
	#[cfg(not(feature = "std"))]
	let duration_since_epoch = Duration::from_secs(
	self.highest_seen_timestamp.load(Ordering::Acquire).saturating_sub(7200) as u64
	);
	self.pending_outbound_payments.remove_stale_payments(
	duration_since_epoch, &self.pending_events
	);

[TheBlueMatt]

We can avoid having to fix this issue if we do #3100 and uncouple the LDK no-std feature from rust-bitcoin no-std.