* support draining feature server manually
* support draining feature server manually
* support CLI to add or remove feature server
* wip
* wip
* wip
* add redis key for feature server and integration test
* wip
* Add envvar support for default expiration interval
* Move envvars to config file, finish work, and add description of envvars to Readme
* Set expires value to MIN_EXPIRES when the retrieved value is lower than it or not parseable