only call curl_global_cleanup once

[jmklix]

Issue #, if available: https://github.com/aws/aws-sdk-cpp/issues/2538

Description of changes: only call curl_glabal_cleanup once

Check all that applies:

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

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