Blackhole.consume(Object) retains consumed objects

Regression. Before CODETOOLS-7901735, Blackhole used to clear its sinks on each iteration. Combined with the usual @State object lifecycle and exponential backoff in consume(Object) method, that meant Blackhole accepted a few objects on warmup iterations, and by the time measurement happened it never captured the objects.

With CODETOOLS-7901735 it changed, and how Blackhole is instantiated for each iteration, capturing objects. This should be fixed back.