Route updates are delayed with large number of services

[galen0624]

We have a large number of services > 2500. The watchBackend func is taking over 15-20 seconds due to the sequential service queries. I am proposing an update to the below code to be a go func with worker pools.

https://github.com/fabiolb/fabio/blob/master/registry/consul/service.go#L57

I will PR this hopefully early next week.

[galen0624]

Closing with #564

[magiconair]

I'll keep this open as the issue which is the motivation for the PR.