Block the mon update removing a preimage until upstream mon writes

Block the mon update removing a preimage until upstream mon writes

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.

Thus, here, we enforce the correct ordering by blocking the
downstream `ChannelMonitorUpdate` until the upstream one completes.
Like the `PaymentSent` event handling we do so only for the
`revoke_and_ack` `ChannelMonitorUpdate`, ensuring the
preimage-containing upstream update has a full RTT to complete
before we actually manage to slow anything down.