implement dns resolution of hostnames at startup

Hostnames already resolved, but only to a single IP address.

This implements a pre-resolve step prior to calling server_new() that expands any hostnames to their resultant IP addresses and calls server_new() for each address.

ie:

```
cluster loadbalanced any_of loadbalancers.example.com ;
```

would expand to:

```
cluster loadbalanced
    any_of
        192.168.1.10:2003
        192.168.1.11:2003
        192.168.1.12:2003
    ;
```