Cleanup stategen pkg

[terencechain]

What type of PR is this?

Clean up

What does this PR do? Why is it needed?
As we are heading towards further audits / mainnet, it's important we go through each package and perform the necessary clean ups. This PR cleans up the stategen pkg

• Removed unnecessary hot and cold abstractions. Instead of loadHotStateByRoot, it's just loadStatebyRoot
• Removed a few unused functions ComputeStateUpToSlot, processStateUpTo, and archivedState... etc
• Moved functions from hot.go and cold.go under getter.go and setter.go as helpers
• There's no logic changes here, this is just renaming and move stuff around

Which issues(s) does this PR fix?

N/A

Other notes for review
Tested:
1.)Validator run time
2.)Syncing from genesis to head using 32 slots archived point
3.)The following script

#!/bin/bash
for i in {0..2000}
do
   time curl -X GET "http://localhost:3051/eth/v1alpha1/validators/assignments?epoch=$i" -H "accept: application/json" | jq . | grep epoch
done

[codecov]

Codecov Report

Merging #7127 into master will decrease coverage by 0.28%.
The diff coverage is 67.30%.

@@            Coverage Diff             @@
##           master    #7127      +/-   ##
==========================================
- Coverage   61.95%   61.67%   -0.29%     
==========================================
  Files         410      406       -4     
  Lines       32826    32109     -717     
==========================================
- Hits        20338    19803     -535     
+ Misses       9618     9489     -129     
+ Partials     2870     2817      -53     

[rauljordan]

Can this be nil?

[terencechain]

It's gated here https://github.com/prysmaticlabs/prysm/blob/master/beacon-chain/state/stategen/getter.go#L153

[rauljordan]

Maybe rename this file to metrics.go for consistency