Validator connection management

[clostao]

Description

A validator should be drop out from validator list when MaxMissedEpochs have passed with no blocks signed by this validator. Should be reconnected when it emits a add_validator_again unsigned extrinsic.

Implements the logic for approving a new validator.

Types of changes

What types of changes does your code introduce?

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)