Split concuerror_scheduler into parts

[aronisstav]

Concuerror's 'scheduler' module currently contains code for a large number of features. The following functionalities should be grouped better and split out in several modules:

• Planning of schedulings: Given a complete execution and a collection of states, sleep sets and wakeup trees, a DPOR algorithm can plan more schedulings. A "planner" component can plan schedulings. The first scheduling should be explored 'separately'.
• Execution of schedulings: A request for a scheduling (or a set of schedulings) can be described by a prefix of an existing scheduling, a sleep set and/or a wakeup tree. A "scheduler" component can be split out to execute a scheduling given such info.
• Logging of errors: After a scheduling has been executed, if errors have been found, a report can be generated. This should happen without communication with any other component.

Execution of schedulings and logging of errors can be combined into a 'scheduler'. The rest belongs into a planner.