ZeroMQ's IPv6 support isn't enabled by default, resulting in
"No such device" errors when attempting to listen on an IPv6
address. This change adds a ipv6 option to the ZeroMQ module
and enables it by default. Further, adds a test configuring
everything to listen on IPv6 ::1 as well, and one test to provoke
the original error. This also regularizes some error messages.
The addr_to_uri() calls weren't actually needed, but they apparently do
not hurt and the result is easier on the eyes, so use them :-)
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.
