mon: Monitor: do not reopen MonitorDBStore during conversion

mon: Monitor: do not reopen MonitorDBStore during conversion

We already open the store on ceph_mon.cc, before we start the conversion.
Given we are unable to reproduce this every time a conversion is triggered,
we are led to believe that this causes a race in leveldb that will lead
to 'store.db/LOCK' being locked upon the open this patch removes.

Regardless, reopening the db here is pointless as we already did it when
we reach Monitor::StoreConverter::convert().

Fixes: #5640
Backport: cuttlefish

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>