When etcd is down today we don't specifically handle the error involved,
which means clients get a generic 500 error. This commit adds a formal
error type internally for both WatchExpired and EtcdUnreachable, and
then converts them to api/errors before returning to the client. It also
upgrades the client to retry on any 429 or 5xx error that has a
Retry-After header, instead of just 429.
In combination, this allows the apiserver to exert backpressure on
controllers that are hotlooping. Picked 2 seconds by default, but we
could potentially ramp that up even further in a future iteration.
Allows us to define different watch versioning regimes in the future
as well as to encode information with the resource version.
This changes /watch/resources?resourceVersion=3 to start the watch at
4 instead of 3, which means clients can read a resource version and
then send it back to the server. Clients should no longer do math on
resource versions.
