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.

mercurial/keepalive.py

@@ -166,7 +166,9 b' class ConnectionManager:'
         if host:
             return list(self._hostmap[host])
         else:
-            return dict(self._hostmap)
+            return dict(
+                {h: list(conns) for (h, conns) in self._hostmap.items()}
+            )
 
 
 class KeepAliveHandler: