When using the new generic P2P transport to open an outgoing connection
to a peer, this will hold the pid of the git-annex-p2p-<netname>
command.
closeConnection simply waits for it. Rather than relying on garbage
collection of the closed handles to close it.
In Remote.Helper.Ssh, connProcess is set to Nothing, even though there
is a similar process being used there. That code stores the pid in
OpenConnection instead, and handles waiting for it itself. A bit ugly,
but not worth cleaning up at this point, maybe later.
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