Rework network events logic organization

[Shatur]

Server and client events logic is separated by "systems for a client event' and "systems for a server event". The big events refactor #262 didn't change it. But I think it makes sense to revisit this aspect as well.

Many users have separate client and server. And they expect to disable ClientEventsPlugin on server and ServerEventsPlugin on client. But this won't work because disabling plugin disables the events entirely.

We need to move all server systems into ServerEventsPlugin and all client systems into ClientEventsPlugin. And events registry should be initialized by the core plugin.