Use Genesis File Timestamp

[alexanderbez]

When a genesis file is provided to simulation, use the file's genesis time unless a custom genesis timestamp is provided by flag.

ref: #4750

---

• Targeted PR against correct branch (see CONTRIBUTING.md)

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.

• Wrote tests

• Updated relevant documentation (docs/)

• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md

• Re-reviewed Files changed in the github PR explorer

---

For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #5148 into master will decrease coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #5148      +/-   ##
==========================================
- Coverage   54.98%   54.95%   -0.03%     
==========================================
  Files         297      297              
  Lines       18225    18231       +6     
==========================================
- Hits        10021    10019       -2     
- Misses       7466     7474       +8     
  Partials      738      738