Improve metrology files format

[jbbarth]

The current format and naming scheme suffers from the following problems:

• activities are identified by their name, which are not guaranteed to be unique (names can be reused for retries) ; it feels wrong since it can be the source of consistency bugs ; I'd like a primary key that's easier to use and offer stronger guarantees (scheduled_event_id or started_event_id are ok)
• the aggregated file maps on simpleflow.swf.history.History.activities structure, which by construct removes retries informations ; and it's not easy to use, it's a list of lists, with on each sublist the name of the activity, and the attributes ; it doesn't merge simply with the standard SWF structure or Webflow's one (which is just an extension of SWF one)

Last, I think the single files and the aggregated one can be simplified down to something exploitable in Webflow, and they don't need to contain extra informations that cannot be remapped to the SWF history trivially.