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>
mon: Monitor: StoreConverter: sanitize 'store' pointer on init

We are supposed to have umount'ed the store and set the pointer to NULL.
We should not tolerate any other case on init().

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: Monitor: StoreConverter: clearer debug message on 'needs_conversion()'

The previous debug message outputted the function's name, as often our
functions do.  This was however a source of bewilderment, as users would
see those in logs and think their stores would need conversion.  Changing
this message is trivial enough and it will make ceph users happier log
readers.

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