Introducing caching

Fix #832 #752

Adds caching capabilities to the converter. Remote calls to the
registry, Schema Parsing and toString were very expensive operations
that are now being cached

Adds a decorator to the registryClient that allows caching of remote
invocations.

Fixing review comments

spring-cloud-stream-core-docs/src/main/asciidoc/spring-cloud-stream-overview.adoc

@@ -1577,6 +1577,13 @@ A client for the Spring Cloud Stream schema registry can be configured using the
   }
 ----
 
+[NOTE]
+====
+The default converter is optimized to cache not only the schemas from the remote server but also the `parse()` and `toString()` methods that are quite expensive.
+Because of this, it uses a `DefaultSchemaRegistryClient` that does not caches responses. If you intend to use the client directly on your code, you can request a bean that also caches responses to be created.
+To do that, just add the property `spring.cloud.stream.schemaRegistryClient.cached=true` to your application properties.
+====
+
 ==== Avro Schema Registry Client Message Converters
 
 For Spring Boot applications that have a `SchemaRegistryClient` bean registered with the application context, Spring Cloud Stream will auto-configure an Apache Avro message converter that uses the schema registry client for schema management.