Fix: Relay daemon ownership and reference counting

The ownership and reference counting of the relay daemon is unclear and
buggy in many ways. It is the cause of memory corruptions, double-free,
leaks, segmentation faults, observed in various conditions.

Fix this situation by introducing a clear ownership and reference
counting scheme for this daemon.

See doc/relayd-architecture.txt for details.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>