This commit Arc-ifies the reference to OffersHandler, which is necessary
because once we set up the grpc server, we'll need a second thread-safe
reference to it.
We update to ldk v20 for the latest blinded route updates, namely being
able to create a blinded route that's more than one hop. On top of that we use
a custom versoin with a small change, which we need in order to send an onion
message along a specified path for LNDK's integration tests:
lightningdevkit/rust-lightning#2868
Before we can send an invoice request, LNDK's onion messenger needs to be fully
started and running. We use a channel close to signal when the messenger is
ready.
To allow external callers to shutdown lndk, we provide a shutdown
trigger and listener to the onion messenger, replacing the single use
channels that we previously used. This is useful for itests, so that
we can cleanly shut down and for callers of the library to manage
execution.
