Further refine Riak KV health check
Further refine Riak KV health check

Change health check to have both a lower and upper threshold. The
health check will fail whenever a message queue surpasses the upper
threshold, and then will continue to fail until all queues are below
the lower threshold. This prevents constantly transitioning back and
forth from healthy to non-healthy.

Add output messages that print out when the health check transitions
from health/non-healthy or non-healthy/healthy.

andytill/riak_kv