LSPS2: Expiring of requests

[johncantrell97]

There is this concept of valid_until in the opening fee params that specifies a datetime when this particular jit channel request becomes invalid.

We currently are not expiring requests. We are only checking the validity at the time the buy request is made. At a minimum we need:

1. A way to poll or regularly check if requests we are waiting on payment for have expired.
2. When htlc_interception happens we should make sure the request is still valid

[tnull]

As of lightningdevkit/rust-lightning#3436 we now prune expired requests on peer disconnection and on block connection.