ctld: Convert struct ctld_connection to a C++ class

Note that some of the functions for handling iSCSI login, discovery,
and kernel handoff are now functions of this class as they are logical
operations on an iSCSI connection and need access to various members
of this class.

This also fixes some memory leaks as ctld_connection wasn't properly
torn down once a connection finishes. These leaks were harmless in
practice since the ctld process exits after handling each connection.

Sponsored by: Chelsio Communications
Pull Request: https://github.com/freebsd/freebsd-src/pull/1794