MINOR: connection: Do not retrieve src/dst on another thread group's FD

MINOR: connection: Do not retrieve src/dst on another thread group's FD

conn_get_src() and conn_get_dst() lazily fetch the connection's
addresses with getsockname()/getpeername() when they were not known
yet. They may be called on a foreign connection by observability code
running on any thread, for example "show peers", which dumps the peers
sessions' connections from whatever thread serves the CLI. With
per-thread-group FD tables, the FD of a connection owned by another
group is not usable from the calling thread.
So just fail if we attempt to access a connection that is owned by
another thread group.