KNOWN_BUGS #17 fixed. A DNS cache entry may not remain locked between two

curl_easy_perform() invokes. It was previously unlocked at disconnect, which
could mean that it remained locked between multiple transfers. The DNS cache
may not live as long as the connection cache does, as they are separate.

To deal with the lack of DNS (host address) data availability in re-used
connections, libcurl now keeps a copy of the IP adress as a string, to be able
to show it even on subsequent requests on the same connection.