Add infra to block `ChannelMonitorUpdate`s on forwarded claims

Add infra to block `ChannelMonitorUpdate`s on forwarded claims

When we forward a payment and receive an `update_fulfill_htlc`
message from the downstream channel, we immediately claim the HTLC
on the upstream channel, before even doing a `commitment_signed`
dance on the downstream channel. This implies that our
`ChannelMonitorUpdate`s "go out" in the right order - first we
ensure we'll get our money by writing the preimage down, then we
write the update that resolves giving money on the downstream node.

This is safe as long as `ChannelMonitorUpdate`s complete in the
order in which they are generated, but of course looking forward we
want to support asynchronous updates, which may complete in any
order.

Here we add infrastructure to handle downstream
`ChannelMonitorUpdate`s which are blocked on an upstream
preimage-containing one. We don't yet actually do the blocking which
will come in a future commit.