feat: Optimize delete asset memory usage

[sumslim]

As part of issue: #81
Optimizing memory usage for cancelFn is delete asset from ES using
Cancel function within the context timeout:
After the specified timeout, the context for the task automatically expires, and the cancellation function is invoked.

[sumslim]

Test case failing due to race condition detected, possibly due to an edge case:
We need to ensure that no operation modifies cancelFnList between appending a cancel function and calling removeCancelFnByIndex. Might need to use mutex to synchronize access to the cancelFnList, but this might bring contention overhead
Using sync.Map as an alternative as it is supposed to provide better performance under high contention
cc @haveiss @irainia

[coveralls]

Pull Request Test Coverage Report for Build 11070490716

Details

• 10 of 14 (71.43%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.04%) to 84.116%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/asset/service.go	10	14	71.43%

Totals
Change from base Build 10676380823:	-0.04%
Covered Lines:	6911
Relevant Lines:	8216

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💛 - Coveralls

[irainia]

Let me know if a separate discussion is required

[irainia]

LGTM! Please don't forget to test. Approving it.