Fix/challenge issue

[lpoli]

Fixes

1. Added lock between object path calculation and writemarker commit in handler.CommitWrite function
2. Fix database connection leakage
3. Use goroutines to send request to validators
4. Allow blank allocation to be valid.

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 #774 (907f825) into staging (a0f9f20) will decrease coverage by 0.00%.
The diff coverage is 33.33%.

@@             Coverage Diff             @@
##           staging     #774      +/-   ##
===========================================
- Coverage    24.40%   24.40%   -0.01%     
===========================================
  Files           77       77              
  Lines         7829     7830       +1     
===========================================
  Hits          1911     1911              
- Misses        5651     5652       +1     
  Partials       267      267              

Flag	Coverage Δ
Unit-Tests	24.40% <33.33%> (-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/allocation/workers.go	0.00% <0.00%> (ø)
...et/blobbercore/handler/object_operation_handler.go	33.05% <ø> (ø)
code/go/0chain.net/blobbercore/reference/ref.go	0.00% <ø> (ø)
code/go/0chain.net/validatorcore/storage/models.go	65.67% <100.00%> (ø)

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[service-0chain]

Manual system tests [failure] with the following config

config	value
tests skipped	FALSE
system-tests	master
gosdk	NOT OVERRIDDEN
zboxcli	staging
zwalletcli	staging
0chain	staging
blobber	fix/challenge-issue
authorizer	staging
0box	staging
0block	staging
0dns	staging
explorer	master
0proxy	staging
0search	staging

[service-0chain]

Manual system tests [success] with the following config

config	value
tests skipped	FALSE
system-tests	master
gosdk	NOT OVERRIDDEN
zboxcli	staging
zwalletcli	staging
0chain	staging
blobber	fix/challenge-issue
authorizer	staging
0box	staging
0block	staging
0dns	staging
explorer	master
0proxy	staging
0search	staging

[cnlangzi]

[ssardana08]

lgtm

The problem does not exist

Requested changes does not seem necessary