Backport bench tweaks for RowSetGetFindBench and gradle setup for Benchmarks.

[jcferretti]

The additional GC time sampling helps ensure individual iterations don't have GC noise. It shows in the JMH output like this:

[...]
Iteration   4: Gc during iteration: count=0, timeMs=0
2770.689 ms/op
                 Max CPU:       3.800 percent
                 Max free heap: 2090860544.000 bytes
                 Max heap:      2109734912.000 bytes
                 Max threads:   2.000 threads
                 Max used heap: 403989504.000 bytes
[...]