[Benchmarking] Some slides improvements

[Andrea Aime]

Hi all,
after FOSS4G I got some private comments about issues in the
presentation itself and I thought to address them in SVN.

The presentation version I've just committed has 3 changes:
- makes sure all charts within the same set of tests have the
   same Y scale to make them easier to compare as one pages through
   the various results
- added a remark that we observed the CPU being the bottleneck
   during the benchmark. I did not stare at the top/vmstat outputs
   for the whole benchmark run but never seen the CPU being significantly
   under used besides the 1 thread runs. Did MapServer people see the
   same?
- added a comment on the T3 results, now that the chart scales have been
   fixed it's evident that Oracle and SDE were significantly slower in
   that test

Maybe we want to republish the results and have the FOSS4G site use this
last version instead of the one we provided during FOSS4G?
A lot of people will look into the results so I guess we should have the
slides be as accurate and easy to interpret as possible?

Cheers
Andrea