Reuse ObjectMapper in SchemaBuilder & JacksonUtils

Currently, calling `JacksonUtils.toJsonNode(obj)`, `JacksonUtils.objectToMap(obj)` or `SchemaBuilder.toJsonNode(obj)` recreate at least one new instance of Jackson ObjectMapper each time. 

ObjectMapper is an expensive object to create, but is thread-safe. We can see in various other locations within Avro that we reuse the same object, and avoid recreation. 

We can perform the same reuse here - and only create the ObjectMapper once.