Historical stats.
Historical stats.

We now have:

%% Period, Secs, Samples
    [{minute, 1, 60},
     {hour, 4, 900},
     {day, 60, 1440}, % 24 hours
     {week, 600, 1152}, % eight days (computer weeks)
     {month, 1800, 1488}, % 31 days
     {year, 21600, 1464}]. % 366 days

The period is just a name which covers the total period over which we
have samples. Secs is the number of seconds per sample, and samples is
the total number of samples at that resolution we keep.

Change-Id: Id43666e1690b17810ac34d15dc05bde7fd94a6ba
Reviewed-on: http://review.northscale.com/1137
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

daverigby/ns_server,pavel-blagodov/ns_server,ceejatec/ns_server,membase/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,daverigby/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,membase/ns_server,membase/ns_server,nimishzynga/ns_server,mschoch/ns_server,pavel-blagodov/ns_server,ceejatec/ns_server,membase/ns_server,membase/ns_server,pavel-blagodov/ns_server,mschoch/ns_server,mschoch/ns_server,nimishzynga/ns_server,ceejatec/ns_server,daverigby/ns_server,ingenthr/ns_server,vzasade/ns_server,t3rm1n4l/ns_server,vzasade/ns_server,t3rm1n4l/ns_server,pavel-blagodov/ns_server,vzasade/ns_server,ingenthr/ns_server,mschoch/ns_server,membase/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,ingenthr/ns_server,daverigby/ns_server,ceejatec/ns_server,vzasade/ns_server,mschoch/ns_server,mschoch/ns_server,nimishzynga/ns_server,ingenthr/ns_server,nimishzynga/ns_server,daverigby/ns_server,t3rm1n4l/ns_server