Proportional scheduling

[GregUtas]

InitThread and Thread now schedule all threads as the result of #126 and #127. The remaining work items are

• provide a default timewheel
• add a command for creating a timewheel (based on the percentage of time allotted to each faction)
• add a command for switching to another timewheel
• Thread.Select chooses next thread based on timewheel
• InitThread acts as clock interrupt to advance timewheel
• track the time spent in each faction and display it in >sched show
• can apply this design recursively, to an element manager that supervises multiple executables, giving each one a guaranteed percentage of the CPU

Each thread now waits on a per-thread Windows event to be signalled to run; this would probably be implemented using a condition variable on other systems.

• conditions variable supported in C++11; see documentation
• "spurious wakeup" problem means that, when awoken, thread must check that the underlying condition is actually true
• implement SetEvent and WaitEvent using a condition variable: see article