Retain reference to share handle from curl handle

Not keeping a reference will not result in use after free, because
curl protects against it, but it will result in a memory leak,
be

Retain reference to share handle from curl handle

Not keeping a reference will not result in use after free, because
curl protects against it, but it will result in a memory leak,
because curl_share_cleanup() will fail. We should make sure that
the share handle object stays alive as long as the curl handles
use it.

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