Trace Intervals

This MR is for helping us group requests/traces over time, necessary for experimentation.

• The client will generate a value for Interval when it submits a request. Initially Interval is 0. By default, Interval increments by 1 every 10 seconds. The value is configurable with the -i client flag.
• Servers will include the Interval in subsequent child calls
• Servers write the value of Interval to the trace -- specifically as an attribute of the Exec span.

When we calculate trace completion (./process), the results will now group by interval and trigger, e.g. as follows