fix: lttng-relayd: use appropriate RCU locking with hash tables

RCU hash tables used in lttng-relayd use the auto-resize feature, which
is performed by concurrent call rcu worker threads. Therefore, we need
to protect the calls to the hash table with appropriate RCU read-side
locking, and use call_rcu to defer memory reclaim.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>