Implement CachedValidatorsBeaconState

[twoeths]

resolves #1789

Goal

• In case of a slow state transition, looping through validators inside TreeBacked structure takes a significant time
• Create CachedValidatorsBeaconState that cache a regular validators array after the 1st loop, use it in lodestar-state-transition only. Note that when processing epoch, we always have to do the loop to prepare EpochProcess.
• When update/add a validator, update to both the cache and TreeBacked<BeaconState>
• When loop/read, use the cached validators if we have it

Trade off

• We have to use setValidator, addValidator, getValidator api in state transition instead of accessing validators directly. I guess we can enforce it at compile time using Omit<BeaconState, "validators"> or something if we want to.

Performance gain with 100k validators as in Pyrmont/Medalla

• Process 1st epoch: save 500ms - 600ms
• From the 2nd epoch, it only takes around 1100ms per epoch (instead of >=2800ms)
• When we have to process 16 validator exits as in the performance test, the time to process block is reduced from 6000ms to 1400ms
• See more details in the performance tests of lodestar-beacon-state-transition

Test

• Passed spec tests
• Synced Pyrmont successfully

Memory

• When we process a regular block it even does not create a cached validators
• It short-live in state transition only then released

Although the performance is improved, I'd like to have everyone review to see if we have any potential issues with this approach. Thanks!

[twoeths]

main file to review

[qlty-cloud-legacy]

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[twoeths]

replaced by #1925