job pull + source: fix broken connection teardown

Issue #56 shows zombie SSH processes.
We fix this by actually Close()ing the RWC in job pull.
If this fixes #56 it also fixes #6 --- it's the same issue.

Additionally, debugging around this revealed another issue: just
Close()ing the sshbytestream in job source will apparently outpace the
normal data stream of stdin and stdout (URG or PUSH flags?).  leading
to ugly errors in the logs.
With proper TCP connections, we would simply set the connection to
linger and close it, letting the kernel handle the final timeout. Meh.

refs #56
refs #6

cmd/config_job_pull.go

@@ -139,10 +139,10 @@ func (j *PullJob) doRun(ctx context.Context) {
 	j.task.Enter("pull")
 	puller := Puller{j.task, client, j.Mapping, j.InitialReplPolicy}
 	puller.Pull()
+	closeRPCWithTimeout(j.task, client, time.Second*1, "")
+	rwc.Close()
 	j.task.Finish()
 
-	closeRPCWithTimeout(j.task, client, time.Second*10, "")
-
 	j.task.Enter("prune")
 	pruner, err := j.Pruner(j.task, PrunePolicySideDefault, false)
 	if err != nil {