Use nproc to set MAX_WORKERS
Use bigint to represent pg_stat_bgwriter values

Bug fix.  The pg_stat_bgwriter system view itself uses bigint to
represent all values.  test_bgwriter.buffers_backend was particularly
prone to rejecting the values pgbench-tools wanted to store as being out
of range for type integer.
First cut at supporting SVG graphs.

Use dstat for all osdata.

Polish report to highlight useful information.

Remove 3d plot. I never cared for it, and Greg Smith already
acknowledged its limited utility.  Use "minor tics" with GnuPlot where
appropriate. Format large numbers using thousand separators for
legibility.

Also, remove backend_sync column, and change names of some things to
reflect that their an average of averages.  Make max_latency actually
reflect maximum latency, rather than average maximum latency across
several tests.

The increased legibility of the SQL queries within webreport is intended
to facilitate further hacking by others.

Make tabbed display the default.