relayd: track the relay_conn_pipe with the fd-tracker

Create the relayd connection pipe through the
fd_tracker_util_pipe_open_cloexec() function which wraps
utils_create_pipe_cloexec(), but tracks the resulting file descriptor.

The close of the pipe is also performed through the fd tracker.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I6ed0d64c973a454de2e50c7cae006fd546299290