RFC: Rate-limiting the collator staking entry/exit queue and remove the unstaking period since there is no slash enabled

[hackfisher]

This is one of the proposals following the discussion of #1454, it introduces two protocol changes:

1. Introduce Rate-limiting entry/exit queue for collator staking, for simple, there will a maximum staking amount inflow/outflow with collator staking each session(collators election period). This is a design choice aimed at maintaining network security and stability.

• Rate limiting the entry/exit of votes helps to prevent sudden, large-scale exits that could potentially destabilize the network. If a large number of votes were allowed to exit at once, it could significantly reduce the total staking pool, thereby lowering the collators security.
• Even though collator only matters for the liveness of the network, a stable collator set is good the consistent operation of the network.
• In a scenario where votes/collators could freely and rapidly exit and join, it might be possible for malicious actors to coordinate exits in a way that disrupts the network. By rate-limiting exits, it adds a layer of protection against such coordinated attacks, known as griefing attacks, where the goal is to disrupt the normal functioning of the network.
• How to define the maximum amount per session? Find the maximum deposit and choose a number above that, because if the maximum number is lower than that, the deposit will not able to entry/exit staking. (Also need to set a maximum limit for deposit module)
• For simplicity of engineering implementation, the sum for rate-limiting can be reset to zero at the end of each session after the collator set is elected. when stake and unstake(exit), check the rate-limiting for eligibility.

2. Remove the unstaking period since there is no slash enabled

• Slash could be enabled if the collators go offline, but current the protocol choose not to slash to keep simple and thinking that no reward for not producing blocks is enough for slight effects for offline.
• The unstaking period can be removed or changed to a smaller period like (7 days for darwinia, 3 days for crab) since no slash enabled.
• If in future, the slash for offline is considered to be necessary again, the withdrawing period(currently unstaking period) can be re-introduce.
• If the unstaking period is to be removed, then the risk of instability is higher enough, the proposal in step 1 by adding rate-limiting must be implemented.
• A digram for reference