Add storage for `ChannelMonitorUpdate`s in `Channel`s

Add storage for `ChannelMonitorUpdate`s in `Channel`s

In order to support fully async `ChannelMonitor` updating, we need
to ensure that we can replay `ChannelMonitorUpdate`s if we shut
down after persisting a `ChannelManager` but without completing a
`ChannelMonitorUpdate` persistence. In order to support that we
(obviously) have to store the `ChannelMonitorUpdate`s in the
`ChannelManager`, which we do here inside the `Channel`.

We do so now because in the coming commits we will start using the
async persistence flow for all updates, and while we won't yet
support fully async monitor updating it's nice to get some of the
foundational structures in place now.