UDP write() accepts hostnames rather than IPs

[itamarst]

Or, to put it another way if we do want to allow it, doesn't use pluggable
resolver IIRC. But that would be bad, we kinda assume packets get sent
immediately (we have exceptions for message length, and this helps keep them in
order and so on) and using resolver would add lots of overhead.

At the very least we need to switch to using twisted resolver and document that
sending unresolved names is expensiive. Alternatively, add depreaction warning
for non IP addresses.

[exarkun]

Resolving hostnames is easy.  I think we should deprecate non-IP addresses and
raise a ValueError in some future version.

[itamarst]

and before 1.3.

[itamarst]

write() no longer accepts hostnames as of 1.3.