Subscribe every WebSocket client to a unique topic, by default under
zeek/cluster/websocket/client/<identifier>/
Add tests that verify that WebSocket clients receive messages on these topics
even if they didn't explicitly pass them in their handshake message.
This is somewhere between feature and bug fix. It aids the ZeroMQ backend
implementation: A WebSocket client that doesn't provide any subscriptions
and immediately starts publishing would discard events until receiving
other nodes subscriptions from the central XPUB/XSUB proxy. ZeroMQ does sender
side topic filtering. When using subscriptions, the client waits until
its own subscriptions are returned from the central XPUB/XSUB proxy,
thereby also learning about other node's subscriptions.
Also, make the no-subscriptions.zeek test use 32 clients sequentially to
trigger potential issues more quickly.
Explicitly notify the internal thread about the shutdown via the
inproc socket pair. This ensures that the internal thread processes
all previous messages on the inproc socket before terminating.
This fixes the scenario where a backend is created, a few messages published
and then immediately terminated as can be done with WebSocket clients.
Previously, some of the messages published might have still been in the
inproc socket's queue and were simply discarded.
Adds the same test for Broker and ZeroMQ backends.
