Refactor tidy after #1863. Add NaturalKeyEntrySimple for single property natural key lookup.

src/main/java/io/ebeaninternal/api/NaturalKeyEntrySimple.java

@@ -0,0 +1,20 @@
+package io.ebeaninternal.api;
+
+class NaturalKeyEntrySimple implements NaturalKeyEntry {
+
+  private final Object key;
+
+  NaturalKeyEntrySimple(Object key) {
+    this.key = key;
+  }
+
+  @Override
+  public Object key() {
+    return key;
+  }
+
+  @Override
+  public Object getInValue() {
+    return key;
+  }
+}