The last resolved address cache uses an index that is mapped to an address
family (AF_INET and AF_INET6).

This commit adds a conversion function from af to index and change the code to
use that all the time only.

In the process, this commit fixes a bug that the last resolved address
accessors were using the af value insted of the index.

Spotted by nickm during review

Signed-off-by: David Goulet <dgoulet@torproject.org>