test: add DebugNonSafepoints to FlightRecorderProfiler

If using JDK 1.8u40+ (or so), then -XX:+UnlockDiagnosticVMOptions
-XX:+DebugNonSafepoints (http://hirt.se/blog/?p=609) would increase
JFR resolution.

closes #622

ubenchmark/src/main/java/org/postgresql/benchmark/profilers/FlightRecorderProfiler.java

@@ -15,9 +15,10 @@
import org.openjdk.jmh.results.Result;

import java.io.File;
import java.util.Arrays;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.TimeUnit;

/**
@@ -44,10 +45,17 @@

    long duration =
        getDurationSeconds(params.getWarmup()) + getDurationSeconds(params.getMeasurement());
    return Arrays.asList(
        "-XX:+UnlockCommercialFeatures", "-XX:+FlightRecorder",
        "-XX:StartFlightRecording=settings=profile,duration=" + duration + "s,filename="
            + params.getBenchmark() + "_" + sb + ".jfr");
    List<String> opts = new ArrayList<>();
    opts.add("-XX:+UnlockCommercialFeatures");
    opts.add("-XX:+FlightRecorder");
    if (!System.getProperty("java.version").startsWith("1.7")) {
      // DebugNonSafepoints requires java 1.8u40+
      opts.add("-XX:+UnlockDiagnosticVMOptions");
      opts.add("-XX:+DebugNonSafepoints");
    }
    opts.add("-XX:StartFlightRecording=settings=profile,duration=" + duration + "s,filename="
        + params.getBenchmark() + "_" + sb + ".jfr");
    return opts;
  }

  private static long getDurationSeconds(IterationParams warmup) {