POC: immutablejs for CachedValidatorsBeaconState

[twoeths]

just want to have some statistics regarding ImmutableJS as @wemeetagain 's ideas in last meeting

• this is based on Implement CachedValidatorsBeaconState #1816. The difference is we use ImmutableJs for the validators cache and apply CachedValidatorsBeaconState globally across lodestar instead of just state transition.
• the statistic

branch	average epoch transition(ms)	time to sync to slot 10000(mins)
master	4114	49
this branch	1395	27

• I really don't see any differences in terms of heap memory in my local environment. Perhaps someone can try in your environments too!

[dapplion]

Oh mama, those results are great!

[twoeths]

replaced by #1925