sshpeer: add a develwarning if an sshpeer is not closed explicitly
The warning is disabled until the next commit, because fixing it
results in a noisy diff due to indentation changes.
Differential Revision: https://phab.mercurial-scm.org/D9998
$ LANG=nonexistent LC_ALL=nonexistent LANGUAGE=nonexistent hg version -q
Mercurial Distributed SCM (version *) (glob)