* Support rate limiting and concurrency limiting CSR requests on servers; handle CA rotations gracefully with jitter and backoff-on-rate-limit in client
* Add CSR rate limiting docs
* Fix config naming and add tests for new CA configs
* Store leaf cert indexes in raft and use for the ModifyIndex on the returned certs
This ensures that future certificate signings will have a strictly greater ModifyIndex than any previous certs signed.
This PR both prevents a blank CA config from being written out to
a snapshot and allows Consul to gracefully recover from a snapshot
with an invalid CA config.
Fixes#4954.
Paul Banks