Also chart JIT/GC metrics during searching

[mikemccand]

We use this StatisticsHelper.java class to watch JIT/GC metrics and print summary to console output at the end of each Indexing and Searching run, but today we only produce a chart for the JIT/GC during indexing (scroll to bottom of that page).

It's a useful chart since it changes quite a bit from JVM to JVM, e.g. the Young GC time dropped substantially recently with the upgrade to OpenJDK 12/13.

It turns out the SearchPerfTest runs already run StatisticsHelper, so all we need to do here is fix the makeGraphs method in nightlyBench.py to parse/aggregate metrics and write a new chart ... shouldn't be too hard.