Use a struct to track MPP parts pending claiming

Use a struct to track MPP parts pending claiming

When we started tracking which channels had MPP parts claimed
durably on-disk in their `ChannelMonitor`, we did so with a tuple.
This was fine in that it was only ever accessed in two places, but
as we will start tracking it through to the `ChannelMonitor`s
themselves in the coming commit(s), it is useful to have it in a
struct instead.