mon: allow leveldb logging

'mon leveldb log = filename'

Signed-off-by: Sage Weil <sage@inktank.com>
osd: optionally enable leveldb logging

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #261 from ceph/wip-leveldb

Reviewed-by: Samuel Just <sam.just@inktank.com>