fix memory leak in peformance tests

[sbiscigl]

Description of changes:

Our CI jobs are failing infrequently with a ASAN check

==192==ERROR: LeakSanitizer: detected memory leaks
--
 
Direct leak of 2808 byte(s) in 27 object(s) allocated from:
#0 0x7fd90482657d in malloc (/lib64/libasan.so.4+0xd857d)
#1 0x7fd90395f1bf in Aws::Malloc(char const*, unsigned long) aws-sdk-cpp/src/aws-cpp-sdk-core/source/utils/memory/AWSMemory.cpp:146
#2 0x4e992a in RequestContext* Aws::New<RequestContext>(char const*) aws-sdk-cpp/src/aws-cpp-sdk-core/include/aws/core/utils/memory/AWSMemory.h:70

this tracks request contexts, storing them in a container, removing them from the map. If for any reason some get orphaned, they will be free-ed during object desctruction.

Check all that applies:

• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
• Checked if this PR is a breaking (APIs have been changed) change.
• Checked if this PR will not introduce cross-platform inconsistent behavior.
• Checked if this PR would require a ReadMe/Wiki update.

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