Store the models compacted so we can store bigger models on zk

We reached the limit of zk for storing LTR models. I want to avoid the
usage of DefaultWrapperModel for as long as possible because we have
deployed in an container orchestrator and this implementation can be
really risky if you can not guarantee the presence of the model on
disk all the time.
So I want to use the managed model feature to upload some bigger
models but zk is dying with OOM. What we noticed is that solr stores
the models in a json indented file. By saving the models in compacted
json our models will be 60% smaller.

user mailing list thread: https://lists.apache.org/thread/5s5dq4j4bd74sfym7frwoqdokoh64s5l