This change attempts to make the stats gathering code more reliable by
not relying on the testbed_stats records to reconstruct usage
statistics.  The main source of errors and total confusion in the
current stats code is that testbed_stats includes all the errors and
transitions, from which I have to reconstruct what happened in order
to determine usage by a project or user.

The new stats code still generates the testbed_stats code, but actual
usage is recorded as it happens, in the experiment_resources table, as
swapins, swapouts, and swapmods occur. Its also much faster to compute
the data for the tables in the web interface, not having to scan a
zillion testbed_stats records in php.

There is a time consuming update to the records that takes place with
a lot of tables locked.