Add option to return max and average data to idlestats.
Add option to return max and average data to idlestats.

Also fix a bug related to start/end times for short-lived experiments.
Note that a side effect of this fix is that the first aggregated data
point returned by idlestats may include data prior to the beginning of
an experiment (when returning data for nodes in an experiment).

nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome