Refactor system event logger and monitor, add new metrics

[talsperre]

This PR removes the notion of a separate global context from system_monitor and system_logger. User's can now implement their event loggers and monitors to make use of the current singleton instead.

Additionally, it adds the graph_info object to the current singleton, but it is not directly exposed to the end user.

[romain-intel]

Something seems to have gotten lost -- there is no graph_info anymore.