Start healthchecks once connection attempt fails

Previously all backends would have healthcheck threads
started immediately. This was mostly useless, as the healthcheck
logic would never mark the backend down - it could only mark it
as back up once it's been marked down my the load-balancing logic.

This generated substantial amounts of traffic.

Now the healthcheck loop should start when the backend is
blacklisted by the load balancing logic and stop once its found
to be back up.
Bump version.
CHANGELOG update to test some pip installation silyness.
sets eventlet version to always be 0.14.0
Merge branch 'enforce_eventlet_version' into sandbox