When we discover we've only partially claimed an MPP HTLC during
`ChannelManager` reading, we need to add the payment preimage to
all other `ChannelMonitor`s that were a part of the payment.
We previously did this with a direct call on the `ChannelMonitor`,
requiring users write the full `ChannelMonitor` to disk to ensure
that updated information made it.
This adds quite a bit of delay during initial startup - fully
resilvering each `ChannelMonitor` just to handle this one case is
incredibly excessive.
Over the past few commits we dropped the need to pass HTLCs
directly to the `ChannelMonitor`s using the background events to
provide `ChannelMonitorUpdate`s insetad.
Thus, here we finally drop the requirement to resilver
`ChannelMonitor`s on startup.
