Store proof-gen time

[lpoli]

Changes

Stores time taken to generate challenge proof.
This PR is re-created as the previous change got reverted due to panic while logging and thus blobber crashing.
Previous PR: #978

Fixes

Tests

Tasks to complete before merging PR:

• Ensure system tests are passing. If not Run them manually to check for any regressions 📋
• Do any new system tests need added to test this change? do any existing system tests need updated? If so create a PR at 0chain/system_test
• Merge your system tests PR to master AFTER merging this PR

Associated PRs (Link as appropriate):

• 0chain:
• gosdk:
• system_test:
• zboxcli:
• zwalletcli:
• Other: ...

[codecov-commenter]

Codecov Report

Merging #989 (f3f77b9) into staging (bacfa51) will not change coverage.
The diff coverage is n/a.

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@@           Coverage Diff            @@
##           staging     #989   +/-   ##
========================================
  Coverage    25.98%   25.98%           
========================================
  Files           76       76           
  Lines         7806     7806           
========================================
  Hits          2028     2028           
  Misses        5506     5506           
  Partials       272      272           

Flag	Coverage Δ
Unit-Tests	25.98% <ø> (ø)

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