Set closed chan mon upd update_ids at creation not application

In c99d3d7 we added a new
`apply_post_close_monitor_update` method which takes a
`ChannelMonitorUpdate` (possibly) for a channel which has been
closed, sets the `update_id` to the right value to keep our updates
well-ordered, and then applies it.

Setting the `update_id` at application time here is fine - updates
don't really have an order after the channel has been closed, they
can be applied in any order - and was done for practical reasons
as calculating the right `update_id` at generation time takes a
bit more work on startup, and was impossible without new
assumptions during claim.

In the previous commit we added exactly the new assumption we need
at claiming (as it's required for the next few commits anyway), so
now the only thing stopping us is the extra complexity.

In the coming commits, we'll move to tracking post-close
`ChannelMonitorUpdate`s as in-flight like any other updates, which
requires having an `update_id` at generation-time so that we know
what updates are still in-flight.

Thus, we go ahead and eat the complexity here, creating
`update_id`s when the `ChannelMonitorUpdate`s are generated for
closed-channel updates, like we do for channels which are still
live.

We also ensure that we always insert `ChannelMonitorUpdate`s in the
pending updates set when we push the background event, avoiding a
race where we push an update as a background event, then while its
processing another update finishes and the post-update actions get
run.

Author: TheBlueMatt