Redis: SLOWLOG command supported

To support the DataDog dashboard out-of-the-box, we need to support a dummy implementation of the SLOWLOG command. It does not need to support returning any actual SLOWLOG entries.

A.C.

Unit/integration tests for both Geode and native Redis passing
README/redis_api_for_geode.html.md.erb updated to make command "supported" (noting limitations!)
or
Stories in the backlog to fix the identified issues (with JIRA tickets) and problem tests ignored