Ignore .tmp files when loading ChannelMonitors in persister

Ignore .tmp files when loading ChannelMonitors in persister

If we are in the middle of persisting an update to a
`ChannelMonitor` when we shutdown (or crash), we'll start up with
a .tmp file lying around. We should ignore it, as failure to
return from the update call should have prevented the
`ChannelManager` from taking any irrevocable action based on the
update.

We're somewhat protected from any filesystem inconsistency behavior
as the `ChannelManager` will refuse to load if we're outright
missing `ChannelMonitor`s.

Fixes #1330.