[reSIProcate] Dns Changes

[Derek MacDonald]

A few changes to DnsResult/DnsInterface:

To avoid race conditions, using DnsInterface is now a takes two steps:

- DnsResult* createDnsResult(DnsHandler* handler=0);
void lookup(DnsResult* res, const Uri& uri);

- SRV lookups now happen when a transport=foo parameter is provided in the
message.
- DTLS SRV lookups, not quite done.
- the Dns provider is now pluggable; the default is Ares.  To add another
Dns provider, implement external/ExternalDns, and change the cxx of
ExternalDnsFactory to create an instance of your class. Note that there is
still a dependency on ares in the internals of DnsResult.

--Derek