use Option for CliqueConfig fields

[Rjected]

Motivation

The CliqueConfig field of a genesis file does not necessarily require both a populated epoch and period field. This results in a ChainConfig like the following to fail to parse:

"config": {
  "clique": {
    "period": 1
  },
  "chainId": 7,
  "homesteadBlock": 0,
  "eip150Block": 0,
  "eip155Block": 0,
  "eip158Block": 0
},

Solution

Change the CliqueConfig fields to be options, and include serde(default) for deserialization.

Add a test for deserializing a genesis.json passed from hive, that only has the period field.

PR Checklist

• Added Tests
• Added Documentation
• Updated the changelog
• Breaking changes