UPG Crash on multi-core setup if Proxy is enabled

Packets expecting `proxy_start_connect_fn` might cause UPG crash if session for the flow has been already deleted in main thread.
Assertion (in case of debug image) or segfault could happen here.
https://github.com/travelping/upg-vpp/blob/master/upf/upf_proxy.c#L367
