Restake to Activate Unstaked Delegate

[envestcc]

What would you like to be added

For delegates who have already been unstaked, provid re-stake and reactivate them.

Specifically, introduce a New Re-SelfStake Transaction, staking at least 1.2M for their own unstaked candidate, continue to reactivate and participate in consensus

Why is this needed

Currently, delegates have the option to Unstake their selfstaking bucket. However, if they wish to continue as delegates, they need to register again through CandidateRegister, which incurs an additional registration fee of 100. It is unreasonable that previously registered names cannot be reused for free.

How important you think this is for IoTeX

• [] must have
• [x] should have
• [] nice to have

Implement Plan

Two implement options:

1. New Re-SelfStake Transaction, staking for their own an unstaked candidate
2. Update CandidateRegister Transaction to compatible re-register case