The cacher blocks requests until it is ready, however, the
ready variable doesn't differentiate if the cacher was stopped.
The cacher is using a condition variable based on sync.Cond to
handle the readiness, however, this was not taking into account
if it was not ready because it was waiting to be ready or it was
stopped.
Add a new condition to the condition variable to handle the
stop condition, and returning an error to signal the goroutines
that they should stop waiting and bail out.
Kubernetes-commit: 2cb3a56e83ae33464edb174b1b6373ba50600759
Antonio Ojea