Make HTTP2PriorityQueue::visitBFS callback safe
Make HTTP2PriorityQueue::visitBFS callback safe

Summary: The callback may delete the node, so defer calling it until the end of the function

Reviewed By: @xiaosophiawang

Differential Revision: D2436735

songfj/proxygen,songfj/proxygen,songfj/proxygen,hiproz/proxygen,pueril/proxygen,Orvid/proxygen,LilMeyer/proxygen,Orvid/proxygen,LilMeyer/proxygen,KublaikhanGeek/proxygen,LilMeyer/proxygen,KublaikhanGeek/proxygen,pueril/proxygen,pueril/proxygen,pueril/proxygen,songfj/proxygen,hiproz/proxygen,Orvid/proxygen,KublaikhanGeek/proxygen,KublaikhanGeek/proxygen,hiproz/proxygen,LilMeyer/proxygen,hiproz/proxygen,Orvid/proxygen