Cleaned up Scheduled Executors. Added Basic test.

Cleaned up and reorganized the scheduled_executor class.
This includes:
 * Renaming member variables to use _ notation.
 * Moving 1-liner functions in the class body.
 * Renaming scheduler_adaptor to scheduling_adaptor.

Also added a basic test for scheduled_thread_pool. This test
simply excutes n closures. Executing 1 closure per second.
The test ensures executing n closures at 1/sec should take
at least n seconds.