mon, osd: Clean up "flush(stringstream); bl.append(stringstream.str())

mon, osd: Clean up "flush(stringstream); bl.append(stringstream.str())

Also: One redundant flush in AuthMonitor, two in PGMonitor, and
a ds << that should have been conditional in Monitor.

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>