[Feature] Blocking queries for /v1/catalog/datacenters

Is it possible to add blocking queries support to the `/v1/catalog/datacenters` endpoint? It'd help a lot for software that uses Consul to get some data (KV/services) across datacenter boundaries (such a Linkerd/Namerd). Instead of re-trying on `HTTP 500` and storming Consul with requests it'd be possible to "patiently wait" until connectivity to the datacenter will recover.