Nest: Forbid adding channels during reconfiguration

When a new channel is found during reconfiguration, do force restart
of the protocol, like with any other un-reconfigurable change.

The old behavior was that the new channel was added but remained in down
state, even if the protocol was up, so a manual protocol restart was
often necessary.

In the future this should be improved such that a reconfigurable
channel addition (e.g. direct) is accepted and channel is started,
while an un-reconfigurable addition forces protocol restart.