keepalive: ensure `close_all()` actually closes all cached connections While debugging why LFS blob downloads are getting corrupted with workers, I noticed that prior to spinning up the workers, the ConnectionManager has 2 connections to the server and calling `KeepAliveHandler.close_all()` left one behind. The reason is the value component of `self._cm.get_all().items()` is a list, and `self._cm.remove()` modifies said list while the caller is iterating over it. Now `get_all()` is a deep copy of both the dict and lists in all cases.

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