Record reason for each switch in given schedule

[derekbruening]

For understanding trace scheduling it would be helpful to have a concise summary of the schedule with the reason for each switch included. The current recorded schedule does summarize where all of the switches occur, but is missing the why. This can be partially inferred by examining trace records prior to the switch, but that is a painstaking process; it would be much nicer to include a reason.

Options for this feature include:

• Augmenting the recorded schedule entries with a reason and bumping the version there
• Adding a new file output by schedule_stats that summarizes the switch locations and reasons. This likely will require refactoring the schedule_stats switch logic today to act on swap-outs instead of swap-ins.