curl.cpp leaks CURL handles

[dgoffredo]

curl_easy_cleanup is not called on all CURL* objects.

curl_easy_cleanup is called (via CurlLibrary::easy_cleanup) when there's a failure to set up the handle (in post), and when the handle is completed or failed (in handle_message). It is not called for handles that have not completed by the time we're shutting down (at the end of run).

Add a call to curl_.easy_cleanup(handle) after this line:

dd-trace-cpp/src/datadog/curl.cpp

Line 508 in 9ac719c

log_on_error(curl_.multi_remove_handle(multi_handle_, handle));