Load BlockInfo fixes required by exec-spec-tests

[rodiazet]

-Add tests cases for BlockInfo loader according to bugs reported.
-Extend from_json implementation for 64 bits integers to support hex and dec strings and values
-Add special case handling to suport currentRandom passed as 0. execution-spec-tests requires it.

[codecov]

Codecov Report

Merging #583 (5668214) into master (dd1b623) will increase coverage by 0.21%.
The diff coverage is 99.03%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #583      +/-   ##
==========================================
+ Coverage   96.85%   97.07%   +0.21%     
==========================================
  Files          73       74       +1     
  Lines        7276     7375      +99     
==========================================
+ Hits         7047     7159     +112     
+ Misses        229      216      -13     

Flag	Coverage Δ
blockchaintests	63.71% <ø> (ø)
statetests	63.11% <70.00%> (+0.01%)	⬆️
unittests	93.88% <99.03%> (+0.26%)	⬆️

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Impacted Files	Coverage Δ
test/statetest/statetest.hpp	100.00% <ø> (ø)
test/unittests/statetest_loader_tx_test.cpp	100.00% <ø> (ø)
test/statetest/statetest_loader.cpp	87.86% <96.00%> (+8.51%)	⬆️
...est/unittests/statetest_loader_block_info_test.cpp	100.00% <100.00%> (ø)
test/unittests/statetest_loader_test.cpp	100.00% <100.00%> (ø)