Initialize ScanningRecipe accumulator key eagerly (#6914)

The recipeAccMessage field used lazy initialization that was not
thread-safe. If two concurrent recipe runs accessed the same shared
ScanningRecipe instance before initialization, both threads could
generate different UUIDs, causing one thread's accumulator data to
be stored under a key that gets overwritten.

Change to eager initialization at field declaration, which is
inherently thread-safe since the JVM guarantees constructor
completion before the object is visible to other threads.

Author: pstreef

rewrite-core/src/main/java/org/openrewrite/ScanningRecipe.java

@@ -35,13 +35,9 @@
  */
 @SuppressWarnings("ALL")
 public abstract class ScanningRecipe<T> extends Recipe {
-    @Nullable
-    private String recipeAccMessage;
+    private String recipeAccMessage = "org.openrewrite.recipe.acc." + UUID.randomUUID();
 
-    private String getRecipeAccMessage() {
-        if (recipeAccMessage == null) {
-            recipeAccMessage = "org.openrewrite.recipe.acc." + UUID.randomUUID();
-        }
+    String getRecipeAccMessage() {
         return recipeAccMessage;
     }