More efficient Exact Belief Propagation

## 🚀 Feature

Requested by @YajunHuang, who is trying to reproduce the [SpEagle algorithm](https://www.andrew.cmu.edu/user/lakoglu/pubs/15-kdd-collectiveopinionspam.pdf) (example pure-Python implementation [here](https://github.com/safe-graph/UGFraud/blob/master/UGFraud/Detector/SpEagle.py)). This is basically exact belief propagation under the BFS order.
@YajunHuang reported that using DGL's `prop_nodes` function is slower than the pure-Python implementation above.