Move EventLogger.updateClient into ServerEngine

This is where it belongs, as the TODO said.
Make EventLogger a class not an object

ServerEngine logically has exclusive ownership of this thing,
so let's make it a local instance available only to ServerEngine.
The logging shim that ServerEngine creates then backends to it.
Merge pull request #86 from sbt/wip/havocp-de-global-event-logger

make EventLogger not global