Update syncPerfGraphs to include line feeds in final log lines.

Also, use context manager to ensure logfile is closed (instead of manually
calling .close() and potentially accidentally leaking a file handler).
Add one more line feed to syncPerfGraph logging.

Ensure that rsync stderr stream is also piped into the logfile.
Merge pull request #920 from thomasvandoren/whitespace-in-sync-perf

Update syncPerfGraphs to include line feeds in final log lines.

Also, use context manager to ensure logfile is closed (instead of manually
calling .close() and potentially accidentally leaking a file handler), and ensure
that stderr from rsync call is included in logfile.