This only changes the script-layer API, but keeps the std::string host
in the C++ layer's ServerOptions. Mostly because the ixwebsocket library
takes host as std::string. Also, maybe at some point we'd want to
support something scheme-based like unix:///var/run/zeek.sock and placing
that in a string could not be totally wrong.
Add tests for IPV6, too.
These test were very sensible to the speed at which ZeroMQ distributes
subscriptions in the cluster and showed to be unreliably when testing with
zeek/btest#113.
The main fix here is to have individual WebSocket clients subscribe to unique
topics, e.g /test/client-0 and /test/client-1, instead of just a shared topic.
This ensures the WebSocket handshake completes only when they observed their
own subscriptions and not prematurely when observing the shared topic.
This seems mainly relevant for tests: In the real world one shouldn't
rely on subscription visibility - you miss messages if you're too late
to the party.
