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.
