mon: allow leveldb logging

'mon leveldb log = filename'

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit c1d5f815546b731e10bfcb81cbcd48b7d432e9c4)
osd: optionally enable leveldb logging

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 0b4c5c1a3349670d11cc3c4fb3c4b3c1a80b2502)
mon: dump MonitorDBStore transactions to file

Signed-off-by: Samuel Just <sam.just@inktank.com>
(cherry picked from commit 797089ef082b99910eebfd9454c03d1f027c93bb)