Don't use reverse DNS for hostname validation

Upon connection we first resolve the hostname and forget about it

https://github.com/apache/mesos/blob/master/3rdparty/libprocess/src/http.cpp#L1462-L1504

then later use reverse DNS on the remote address to get back a hostname

https://github.com/apache/mesos/blob/4708c2a368e12a89669135f47777d0dd05d9b0b2/3rdparty/libprocess/src/posix/libevent/libevent_ssl_socket.cpp#L548-L556

and verify the server certificate against that.

Instead, we should verify the server certificate against the hostname that was used by t he client to initiate the connection.