Maintain MQ threshold per node.
Hardwire specific visibility thresholds.

Local node always gets ~0u (always); dead nodes always get 0 (ie, never).
Only construct cr_mq when we actually want to send messages to the node.

Otherwise, cr_out would just keep piling messages up in a dead node's MQ.
Delete extraneous return statement.
Test and fix handling of chimera nodes.
Fix virtual-bus starting with all firewalls open.