Trend graph performance degrades as more builds are present in the Project.
Implement better caching to make page display more responsive.

One possible approach is to generate the latest graphs when each build completes
and store them in the cobertura directory at the project level (i.e. the same
directory as the painted source files)

Note: this will lose the feature that each build only shows the trend up to that
build, as they will now be showing the latest trend for that package. (This
"feature" may not be as useful as a more responsive page rendering though)