Fix: notification thread: RCU-safe reclaim of hash table nodes

Fix: notification thread: RCU-safe reclaim of hash table nodes

Nodes that are put in a rculfhash hash table created with the
"auto resize" flag need to beware that a worker thread can access the
hash table nodes as a RCU reader concurrently, and that this worker
thread can modify the hash table content, effectively adding and
removing "bucket" nodes, and changing the size of the hash table
index.

Therefore, even though only a single thread reads and updates the hash
table, a grace period is needed before reclaiming the memory holding
the rculfhash nodes.

Moreover, handle_notification_thread_command_add_channel() misses a
RCU read-side lock around iteration on the triggers hash table. Failure
to hold this read-side lock could cause segmentation faults when
accessing hash table objects if a hash table resize is done by the
worker thread in parallel with iteration over the hash table.

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