Support query cut-off

[sherman-the-tank]

There could be power-law vertices existing in the graph. When a query hitting such vertices, the latency and the cpu cost will dramatically increase. We want to limit the impact of such query. One possible solution is to have a degree threshold. Any query will stop accessing more edges/neighbors when it reaches the threshold.

[CPWstatic]

We now have a global flag to implement this requirement, and we will implement it better by pushing down limit in the future.