Factored event queue into its own class and fixed race condition between multiple starting enclaves

[cmnrd]

This PR does two things:

1. Code that is related to managing the event queue is factored out from the Scheduler class and moved to a new EventQueue class. This makes the event queue fully encapsulated and simplifies the scheduler.
2. This fixes a race condition that could occur during startup in enclave programs with cycles. All enclaves initialize their logical time to zero. Only at startup they advance their logical time to the start tag. In conjunction with the tag acquiring mechanism implemented in Enclave connections and enclave coordination #44 this creates a race. Another enclave that started up already could send acquire requests, which leads to empty events being inserted in the event queue. These acquire request, however, might be issued for tags that are earlier than the start tag. In consequence, we might end up with a situation where we advance time to the start tag, but we have (empty) events in the queue that lay in the past. This is fixed by protecting the start method with a mutex and by discarding any empty events that are scheduled before the start tag.

[lhstrh]

Looks good! Well documented also... 👍