BlueStore: Fix a bug when FSCK is invoked in mount()/umount()/mkfs() with DEEP option

BlueStore: Fix a bug when FSCK is invoked in mount()/umount()/mkfs() with DEEP option
Fixes: https://tracker.ceph.com/issues/53185
NCB mishandles fsck DEEP in mount()/umount()/mkfs() case causing it to remove the allocation-file without destaging a new copy (which will cost us a full rebuild on startup)
There are also few confiliting calls to open_db()/close_db() passing inconsistent read-only flag

We fix both issues by storing open-db type (read-only/read-write) and using it for close-db (which won't pass read-only flag anymore)
We also move allocation-file destage to close-db so it will be refreshed after being removed by fsck and such

Signed-off-by: Gabriel Benhanokh <gbenhano@redhat.com>