Prefactor: Simplify state transitions, drop lock around PaymentQueue

The previous transition pattern of `OutboundJITChannelState` was never
great: we'd take `&mut self`, only to also return `Self` and required
updating the state externally to the state transtion methods. In
addition, we previously wrapped `PaymentQueue` in an `Arc<Mutex<..>>` to
avoid cloning them during state transtions.

Here, we clean up all of this, having the state transtion methods
updating the state in-place and merely returning an `action` in the
method's `Result`s. We also use `core::mem::take` to move the
`payment_queue` to the new states without reallocation.

Author: tnull

lightning-liquidity/src/lsps2/payment_queue.rs

@@ -54,6 +54,12 @@ impl PaymentQueue {
 	}
 }
 
+impl Default for PaymentQueue {
+	fn default() -> Self {
+		Self { payments: Vec::new() }
+	}
+}
+
 #[cfg(test)]
 mod tests {
 	use super::*;