osd: make pg removal thread more friendly

For a large PG these are saturating the filestore and journal queues.  Do
them synchronously to make them more friendly.  They don't need to be fast.

Signed-off-by: Sage Weil <sage@inktank.com>
osd: set pg removal transactions based on configurable

Use the osd_target_transaction_size knob, and gracefully tolerate bogus
values (e.g., <= 0).

Signed-off-by: Sage Weil <sage@inktank.com>