use explict field to differentiate slot_since_genesis slot_since_hard_fork

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Explain your changes:
This PR changes the field curr_global_slot to curr_global_slot_since_hard_fork in consensus_state. The purpose is to make it more clear that which kind of slot (since_genesis vs. since_hard_fork) is being used. It should not affect the usage of mainnet config.

Explain how you tested your changes:
*

Checklist:

• Dependency versions are unchanged
  • Notify Velocity team if dependencies must change in CI
• Modified the current draft of release notes with details on what is completed or incomplete within this project
• Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• All tests pass (CI will check this if you didn't)
• Serialized types are in stable-versioned modules
• Does this close issues? List them

• Closes Address json serialization for global slots not differentiating between Since_genesis and Since_hard_fork #14293

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[georgeee]

Lints job failed -- needs to be fixed

This fix might break compatibility of berkeley with ITN, but this is probably fine.

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[deepthiskumar]

!approved-for-mainnet

[deepthiskumar]

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[coveralls]

coverage: 61.527%. first build
when pulling 6ef4d10 on global-slot-json-format
into 16bd00b on berkeley.