RFR: 8255299: Drop explicit zeroing at instantiation of Atomic* objects

[Сергей Цыпанов]

As discussed in https://github.com/openjdk/jdk/pull/510 there is never a reason to explicitly instantiate any instance of `Atomic*` class with its default value, i.e. `new AtomicInteger(0)` could be replaced with `new AtomicInteger()` which is faster:
@State(Scope.Thread)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
@BenchmarkMode(value = Mode.AverageTime)
public class AtomicBenchmark {
  @Benchmark
  public Object defaultValue() {
    return new AtomicInteger();
  }
  @Benchmark
  public Object explicitValue() {
    return new AtomicInteger(0);
  }
}
THis benchmark demonstrates that `explicitValue()` is much slower:
Benchmark                      Mode  Cnt   Score   Error  Units
AtomicBenchmark.defaultValue   avgt   30   4.778 ± 0.403  ns/op
AtomicBenchmark.explicitValue  avgt   30  11.846 ± 0.273  ns/op
So meanwhile https://bugs.openjdk.java.net/browse/JDK-8145948 is still in progress we could trivially replace explicit zeroing with default constructors gaining some performance benefit with no risk.

I've tested the changes locally, both tier1 and tier 2 are ok. 

Could one create an issue for tracking this?

-------------

Commit messages:
 - 8255299: Drop explicit zeroing at instantiation of Atomic* objects

Changes: https://git.openjdk.java.net/jdk/pull/818/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=818&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255299
  Stats: 19 lines in 17 files changed: 0 ins; 3 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/818.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/818/head:pull/818

PR: https://git.openjdk.java.net/jdk/pull/818