When watching all services, consul-template is DOSing the Consul agent

### Consul Template version

```
consul-template v0.19.4 (68b1da2)
```

### Expected behavior

Consul template should rate limit its calls to avoid DOSing the Consul Servers.

### Actual behavior

When using consul-template to watch all services on a large cluster (1k+ kind of services, more than 4500 nodes, Consul Indexes almost always change, especially on /catalog/services), consul-template very quickly DOS the Consul Servers by requesting far too fast to its local agent data and its local agent fowarding it very quickly to Consul Servers. In one of our configuration, we then see more than 500Mb/s data between Consul-Agent and Consul-Server.