osd/PG: wait for commit *and* readable before deleting more of PG

osd/PG: wait for commit *and* readable before deleting more of PG

For filestore, waiting for onreadable ensures that (1) the backend has done
(all) of the deletion work (we are throttled) and (2) that the flush() will
not block.  So, all good.

For bluestore, onreadable happens at queue time, so the flush() was needed
to throttle progress.  However, we don't want to block the op thread on
flush.  And waiting for commit isn't sufficient because that would not
capture the filestore apply work.

Fix by waiting for both commit and readable before doing more deletion
work.

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