consensus: committee sampling

[SebastianElvis]

Problem

We need to implement committee sampling. For now the miners of latest x blocks are elected as the committee

Description

In struct server of https://github.com/SebastianElvis/orazor/blob/main/server.go,

• [x] Add an extra field on committeeAddrs that tracks the committee members
• [x] A committee sampling function that, given the current blockchain, outputs the committee
• [x] In OnBlock event, add the logic: get the best chain and refresh the committee

Alternatives

Additional context