sshpeer: add a method to check if a doublepipe is closed So we can tell in a next commit if we're trying to close an already closed connection or not (in which case, we may warn). Differential Revision: https://phab.mercurial-scm.org/D9996

sshpeer: make sshpeer.close() close the underlying connection So the connection can be closed eagerly in future commits, instead of relying on __del__. Differential Revision: https://phab.mercurial-scm.org/D9995