Thread-safe access to SubscriptionData

[Yadunund]

This PR introduces SubscriptionData class that manages the lifetime of Zenoh artifacts. It makes access to class members thread-safe where possible. Some z_closure callbacks still work with type erased raw ptrs but this will be addressed when we migrate to zenoh-cpp.

It seems like it is not feasible to store rmw_node_t * as type erased void * in rmw_subscription_t->data like we did with rmw_publisher_t->data .

The problem is that because rmw_subscriptions_t in rmw_wait contains an array of the type erased subscriptions handles (ie, rmw_subscription_t->data ), there is no way to get the SubscriptionData::SharedPtr here

rmw_zenoh/rmw_zenoh_cpp/src/rmw_zenoh.cpp

Lines 3220 to 3221 in cf8e8a2

	auto sub_data =
	static_cast<rmw_zenoh_cpp::rmw_subscription_data_t *>(subscriptions->subscribers[i]);

. ie we cannot traverse the heirarchy from rmw_subscription->data->rmw_node_t->context->impl->get_node_data()->get_sub_data().

The only option we have is storing a rawptr to SubscriptionData in rmw_subscription_t->data but leads to the risk of it becoming a dangling pointer when the SubscriptionData::SharedPtr is deleted.

[clalancette]

This generally looks good. I've left some things to improved, and it needs to be rebased. Once those are done, I'll run some more extensive tests on it.

[clalancette]

Is this a separate bugfix?

[Yadunund]

The approach with capturing the weak_ptr ensures that if the event callback executes for some reason after the last reference to PublisherData::SharedPtr goes out of scope, we would safely exit the callback. This is what I want to do with some of the Zenoh callbacks as well once we switch to zenoh-cpp.
I would have liked to also do it within rmw_subscription_event_init but given that we're storing the raw_ptr to SubscriptionData in rmw_subscription_t->data and hence not able to retrieve the shared_ptr to the same, we are stuck with potential to dereference a dangling pointer if the scenario above. But this scenario should never occur given the implementation of rcl*