More explicit management of EventContext in IEventProcessor

This is a little step towards a movable only EventContext (as suggested by @graven some time ago).

The key points are

• separation of the EventContext creation from IEventProcessor::executeEvent
• replace IEventProcessor::executeEvent( void* ) with IEventProcessor::executeEvent( EventContext&& )

The change is backward incompatible, so requires adaptation in the IEventProcessor specializations.

While implementing the change I noticed a few issues in HiveSlimEventLoopMgr which should be fixed, but since LHCb does not use it, it's low priority for me.

⚠ Includes !876 (merged)