Avoid "BUG #3329: Lost orphan ..." during accept problems (#780)

Avoid "BUG #3329: Lost orphan ..." during accept problems (#780)

Comm::TcpAcceptor creates a Comm::Connection with an open FD. Lots of
things could go wrong while that connection object travels to its
intended owner (e.g., Ftp::Server). A Connection object abandoned during
that travel will auto-close, triggering level-4 "BUG #3329" messages.

TODO: The manual enter/leaveOrphanage() tracking is unreliable. We need
to design and implement a true connection ownership concept that does
not require such tracking. These changes highlight handover spots.

Also made a few TcpAcceptor members protected to reduce the chance of
missing a Connection recipient (i.e. TcpAcceptor subscriber). Most of
these members should be (at least) protected for other reasons as well.