os/newstore: avoid sync append for small ios

os/newstore: avoid sync append for small ios

An append is expensive in terms of latency (write, fdatasync, kv commit),
while a wal write is just the kv commit and the write and fdatasync are
async.  For small IOs doing the wal may improve performance.

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