Ignore partially-pruned channels during routing

Ignore partially-pruned channels during routing

If we prune one side of a channel's `ChannelUpdateInfo` that means
the node hasn't been online for two weeks (as they haven't
generated a new `channel_update` in that time). In such cases, even
if we haven't yet pruned the channel entirely, we should definitely
not be treating these channels as candidates for routing.

Note that this requires some additional `channel_update`s in the
router tests, but all of the new ones are added as disabled
channels.

Fixes #1824