Cache the Java serializer lookup per type

[vpaturet]

This fixes issue #1299 by implementing the missing logic in the ExternalizableSerializer cache.

[theigl]

I'm wondering if we should replace this whole lookup with a ClassValue.

	private final ClassValue<JavaSerializer> javaSerializerByType = new ClassValue<>() {
		protected JavaSerializer computeValue(Class<?> type) {
			return isJavaSerializerRequired(type) ? new JavaSerializer() : null;
		}
	};

Then the whole method just becomes:

	private JavaSerializer getJavaSerializerIfRequired (Class type) {
		return javaSerializerByType.get(type);
	}

This is faster than the map lookup and much cleaner.

[vpaturet]

This looks much better! I added one more commit with your suggestion.

[theigl]

Thank you @vpaturet!