Off-chain node selection

[lukewwww]

Since we have introduced QoS weighted probability of node selection, the complexity of the algorithm to select node becomes O(n) instead of O(1), which increases the gas fee significantly when the number of nodes become large.

I think we need a new method to select nodes in O(1) time. The high level idea is to utilize VRF, the blockchain generates a random number, and the nodes could proof to the blockchain that it is the selected node under the given node selection schema.

[lukewwww]

The discussion will be continued in #4