AtomicLongPosition plain methods fix. (#322)

The methods get and set, which have plain semantics, where calling
AtomicLong.get/set which  have volatile semantics.

So they were calling methods with were more thread-safe than needed.

agrona/src/main/java/org/agrona/concurrent/status/AtomicLongPosition.java

@@ -77,7 +77,7 @@ public int id()
      */
     public long get()
     {
-        return value.get();
+        return value.getPlain();
     }
 
     /**
@@ -99,7 +99,7 @@ public long getAcquire()
      */
     public void set(final long value)
     {
-        this.value.lazySet(value);
+        this.value.setPlain(value);
     }
 
     /**