Epoch setup random source update

[tarakby]

This PR is part of improving the security of the epoch source of randomness after updating the epoch core-contract (onflow/flow-core-contracts#397).

• update the core-contract version with the latest epoch contract changes
• node software makes sure that the source of randomness is a hex string of exactly 128 bits and rejects any other string
• add tests for invalid sources

[codecov-commenter]

Codecov Report

Attention: 57 lines in your changes are missing coverage. Please review.

Comparison is base (3c27e97) 55.99% compared to head (d89815f) 55.65%.

Files	Patch %	Lines
utils/unittest/service_events_fixtures.go	0.00%	55 Missing ⚠️
utils/unittest/execution_state.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5088      +/-   ##
==========================================
- Coverage   55.99%   55.65%   -0.35%     
==========================================
  Files         649      994     +345     
  Lines       63379    95050   +31671     
==========================================
+ Hits        35492    52900   +17408     
- Misses      25321    38177   +12856     
- Partials     2566     3973    +1407     

Flag	Coverage Δ
unittests	55.65% <16.17%> (-0.35%)	⬇️

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[tarakby]

This PR will be open once onflow/flow-core-contracts#397 is merged

[joshuahannan]

@tarakby The core contracts PR is merged and is getting upgraded on mainnet tomorrow. Does anything need to be done about this PR?

[tarakby]

@joshuahannan the only constraint is to not deploy this PR on Mainnet till the contract is upgraded. This is guaranteed once the contract is upgraded so I'll open this PR tomorrow.