kaiax/staking: Implement Preload feature

[blukat29]

Proposed changes

• Add the "Preload" feature in the kaiax/staking. Serves the same purpose with the existing PreloadStakingInfo.
• Usage

  mStaking.AllocPreloadRef() -> RefId
  mStaking.PreloadFromState(RefId, StateDB)
  mStaking.FreePreloadRef(RefId)

• Data structure

  PreloadBuffer: [StateRoot] -> { StakingInfo, [RefId] -> struct{} }
  
  Suppose there are two concurrent state reexec threads.
  First thread is assigned RefId=1, and reexec through three blocks of which state roots are H1, H2, H3.
  Second thread is assigned RefId=2, and reexec through three blocks of which state roots are H3, H4, H5.
  The resulting SideStates looks like this:
  
  H1 -> { S1, [1] }
  H2 -> { S2, [1] }
  H3 -> { S3, [1,2] }
  H4 -> { S4, [2] }
  H5 -> { S5, [2] }

• The GetStakingInfo() also refers to the PreloadBuffer.

The SideState feature has two usages.

• Before CreateSnapshot in cn.New() -- this usage will be removed as we introduce kaiax/gov and kaiax/valset. So I simply modified the existing reward.PreloadStakingInfo as a temporary fix.
• In StateAtBlock - properly using StakingModule.

Types of changes

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• [ ] Bugfix
• [x] New feature or enhancement
• [ ] Others

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• [ ] Lint and unit tests pass locally with my changes ($ make test)
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Related issues

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Further comments

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[blukat29]

@hyeonLewis PTAL. Without this PR, the CreateSnapshot at node startup might break.

[blukat29]

@hyeonLewis I thought the Preload feature may be inaccurate because it stores StakingInfo keyed by block numbers. When there is a bad block then block rewind, the block state may differ on the same number. In the process of moving from number-keyed to roothash-keyed, I got to store the state. But on a second thought, while I will keep the roothash-key, but resource-wise it's better to store the StakingInfo than the whole StateDB.

[blukat29]

@hyeonLewis Modified to store pre-calculated StakingInfo instead and tests pass. Let me polish it and fix the "can't be released" issues.