Improper memory management on cancel

[irainia]

Describe the bug
This PR introduced changes, which contains improper memory management. To be exact, this line contains the list of cancel function for every request, as populated by this line

Expected behavior
There should be a mechanism to clean the unused cancel function out of memory.

Additional context
For the first few request, it won't raise any concerns. But, overtime, the memory consumption will be increased. Although the increase may be insignificant.