Update Validity Conditions to Match Spec

[rauljordan]

No tracking issue

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Description

Write why you are making the changes in this pull request
Currently, our block validity conditions had 4 conditions even though the spec on master has 3. One of them was wrong, as we previously had that:

Verify time.Now() > state.genesis_time + block.slot* SLOT_DURATION.

but instead we should have

Verify time.Now() > state.genesis_time + (block.slot-GENESIS_SLOT)* SLOT_DURATION.

Additionally, we were checking that the DepositRoot of the ETH1 data existed in the powchain, but instead we should be checking if BlockHash points to a block.

[codecov]

Codecov Report

Merging #1611 into master will decrease coverage by 0.16%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master    #1611      +/-   ##
==========================================
- Coverage   71.73%   71.56%   -0.17%     
==========================================
  Files          94       94              
  Lines        6630     6465     -165     
==========================================
- Hits         4756     4627     -129     
+ Misses       1465     1433      -32     
+ Partials      409      405       -4