librbd: tweaks to improve throughput for journaled IO

librbd: tweaks to improve throughput for journaled IO

Increase the per-object append flush bytes to 1MiB to mimic the 16MiB
in-memory writeback cache. Finally, default enable the new journal
writethrough until flush option.

Performance results comparing no journal, pre-change, and post-change:

	no journal	original	modified
4K	40310.84	7223.73		27427.27
32K	21530.37	3256.39		10284.05
256K	3159.67		564.73		1627.16
4M	223.03		48.03		91.91

Fixes: http://tracker.ceph.com/issues/40072
Signed-off-by: Jason Dillaman <dillaman@redhat.com>