GOSSIP-25 Create reaper process to expire per-node data

src/main/java/org/apache/gossip/manager/SystemClock.java

@@ -0,0 +1,15 @@
+package org.apache.gossip.manager;
+
+public class SystemClock implements Clock {
+
+  @Override
+  public long currentTimeMillis() {
+    return System.currentTimeMillis();
+  }
+
+  @Override
+  public long nanoTime() {
+    return System.nanoTime();
+  }
+
+}