Use ConcurrentSkipListMap in Model for knowledge

The knowledge cache previously used a synchronized IdentityMap to cache
knowledge indexes. ConcurrentHashMap was not used because
computeIfAbsent needed to be called recursively when creating indexes,
and that doesn't work with ConcurrentHashMap. However,
ConcurrentSkipListMap can be work with recursive calls to
computeIfAbsent.

smithy-model/src/main/java/software/amazon/smithy/model/Model.java

@@ -18,15 +18,16 @@
 import java.util.AbstractSet;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.Comparator;
 import java.util.HashMap;
 import java.util.HashSet;
-import java.util.IdentityHashMap;
 import java.util.Iterator;
 import java.util.Map;
 import java.util.Objects;
 import java.util.Optional;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentSkipListMap;
 import java.util.function.Function;
 import java.util.stream.Stream;
 import software.amazon.smithy.model.knowledge.KnowledgeIndex;
@@ -87,7 +88,7 @@ public final class Model implements ToSmithyBuilder<Model> {
 
     /** Cache of computed {@link KnowledgeIndex} instances. */
     private final Map<Class<? extends KnowledgeIndex>, KnowledgeIndex> blackboard
-            = Collections.synchronizedMap(new IdentityHashMap<>());
+            = new ConcurrentSkipListMap<>(Comparator.comparing(Class::getCanonicalName));
 
     /** Lazily computed trait mappings. */
     private volatile TraitCache traitCache;