Another fix for ClientData.

[clalancette]

In this fix, what we do is to manually allocate a struct that wraps the ClientData. Inside of that struct, we take a shared_ptr reference to ClientData. And then in client_data_drop, we manually delete the struct.

This ensures that the ClientData object is not destroyed while there are still requests in-flight. This also allows us to get rid of the tricky tracking of the num_in_flight; we don't need it anymore, and the rmw_node_data_t can just drop the reference directly.

Finally, I'll note that this is basically equivalent to do a lambda capture when zenoh-cpp. Still, I think this is worthwhile getting in before #327 since it fixes UB and it is relatively easy to understand.

[Yadunund]

This is a much cleaner approach with the existing implementation!