mon: dump MonitorDBStore transactions to file

Signed-off-by: Samuel Just <sam.just@inktank.com>
ceph-monstore-tool: add MonitorDBStore trace dumper

Signed-off-by: Samuel Just <sam.just@inktank.com>
ceph-monstore-tool: added replay
mon: set MonitorDBStore options on open

No idea why.

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