Avoid potential duplicate parse operation within GenericJackson2JsonRedisSerializer.

[christophstrobl]

In order to read a potential type key json source might be parsed multiple times which imposes a computation overhead. This could be avoided by reusing already parsed JsonNodes as outlined in https://github.com/spring-projects/spring-data-redis/pull/2999/commits/8aaf501b73a8c74df7db839793d55b6e83720835.