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* Reduce allocations due to Jackson serialization. This patch attacks two sources of allocations during Jackson serialization: 1) ObjectMapper.writeValue and JsonGenerator.writeObject create a new DefaultSerializerProvider instance for each call. It has lots of fields and creates pressure on the garbage collector. So, this patch adds helper functions in JacksonUtils that enable reuse of SerializerProvider objects and updates various call sites to make use of this. 2) GroupByQueryToolChest copies the ObjectMapper for every query to install a special module that supports backwards compatibility with map-based rows. This isn't needed if resultAsArray is set and all servers are running Druid 0.16.0 or later. This release was a while ago. So, this patch disables backwards compatibility by default, which eliminates the need to copy the heavyweight ObjectMapper. The patch also introduces a configuration option that allows admins to explicitly enable backwards compatibility. * Add test. * Update additional call sites and add to forbidden APIs.