Multi-model-server

* Upgrade [log4j2](https://logging.apache.org/log4j/2.x/security.html) version to 2.16.0 to address [CVE-2021-44228](https://www.lunasec.io/docs/blog/log4j-zero-day/) and [CVE-2021-45046](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-45046)
* Updated logging docs to address migration from log4j v1 to v2
* Code fix for setting `preload_model` default as null for register model request
* Fix channel closures in ModelServerTest

Allows custom HTTP status in mms.service.Service to be returned to client

This release contains minor fixes to bump up Netty & Log4j versions.

This release contains minor fixes to make sure resource cleaning is done for terminated worker threads
* Terminates the STDOUT and STDERR ReaderThreads for a Worker when it is scaled down

This release contains API changes and fixes to make sure resource cleaning is handled synchronously.
* Load model API sends a conflict response instead of a bad request response when trying to register an already registered model. 851
* Unregister model API is now synchronous and will wait until all resources are cleaned before sending a response back. A timeout feature was also added to config if users don't want to wait. 853