Make Makefile idomatic

Previously the Makefile for starting and stopping the Redis and
sentinel instances wasn't particularly idiomatic.  Rather than having
targets based on created files, it used a lot of .PHONY targets.  This
change makes the Makefile idiomatic for everything but stopping, and
the tests and points to the way of migrating it into a Gradle build,
eliminating the need for the Makefile altogether.