In brief, this commit:
* Updates to the latest upstream 'cookie', fixing a memory leak.
* Make changes to 'CookieJar' observable only through 'pending()'.
* Deprecates 'Client::new()' in favor of 'Client::tracked()'.
* Makes 'dispatch()' on tracked 'Client's synchronize on cookies.
* Makes 'Client::untracked()' actually untracked.
This commit updates to the latest 'cookie' which removes support for
'Sync' cookie jars. Instead of relying on 'cookie', this commit
implements an op-log based 'CookieJar' which internally keeps track of
changes. The API is such that changes are only observable through
specialized '_pending()' methods.
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