client: use con-based send_message

Signed-off-by: Sage Weil <sage@inktank.com>
common/LogClient: use con-based send_message

Signed-off-by: Sage Weil <sage@inktank.com>
osd: use Connection::send_message()

Signed-off-by: Sage Weil <sage@inktank.com>