fix(challenge): load challenges one by one from db to prevent oomkilled

[cnlangzi]

Changes

• mark challenge as Cancelled if it is timeout by cct

Fixes

• fixed panicking issue if there are a lot of waiting challenges in db.

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 #779 (6ec40b3) into staging (b0bcbb3) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           staging     #779      +/-   ##
===========================================
- Coverage    24.41%   24.40%   -0.01%     
===========================================
  Files           77       77              
  Lines         7826     7829       +3     
===========================================
  Hits          1911     1911              
- Misses        5648     5651       +3     
  Partials       267      267              

Flag	Coverage Δ
Unit-Tests	24.40% <0.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...de/go/0chain.net/blobbercore/writemarker/worker.go	0.00% <0.00%> (ø)

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[peterlimg]

lgtm

fixed