common/options: bluefs_buffered_io=true by default

common/options: bluefs_buffered_io=true by default

Enable bluefs_buffered_io again because it makes a huge user-visible
improvement in metadata intensive scenarios, such as but not limited to
PG deletion.

In our environment, deleting PGs from 4 hybrid OSDs (sharing one SATA SSD block.db) saturates
the block.db at 350MB/s reads and causes slow reqs and flapping on the OSDs.
Those OSDs have 3GB osd_target_memory.
Enabling bluefs_buffered_io drops the SSD IO down to <1MBps and the OSDs
are performant again. (The underlying PG deletion inefficiency is being
solved separately, but the page cache is so much more effective than
the bluestore cache in this scenario).

Lastly, remove the comment about swap. We should separately advise
operators to disable swap on OSD machines, as it is much better in
our experience to OOM and restart than to chug along swapping.

Signed-off-by: Dan van der Ster <daniel.vanderster@cern.ch>
Related-to: https://tracker.ceph.com/issues/45765
Related-to: https://tracker.ceph.com/issues/47044