* origin/topic/awelzel/4586-zeromq-ipv6:
cluster/zeromq: Short-circuit DoPublishLogWrite() when not initialized
cluster/zeromq: Hook up and enable IPV6 by default
cluster/zeromq/connect: Make failures fatal
cluster/zeromq: Move log_push creation to DoInit()
(cherry picked from commit cab4ebf513)
This is a cluster backend implementation using a central XPUB/XSUB proxy
that by default runs on the manager node. Logging is implemented leveraging
PUSH/PULL sockets between logger and other nodes, rather than going
through XPUB/XSUB.
The test-all-policy-cluster baseline changed: Previously, Broker::peer()
would be called from setup-connections.zeek, causing the IO loop to be
alive. With the ZeroMQ backend, the IO loop is only alive when
Cluster::init() is called, but that doesn't happen anymore.
