In a multi-pool setup when disks are coming up, or in a single pool
setup let's say with 100's of erasure sets with a slow network.
It's possible when healing is attempted on `.minio.sys/config`
folder, it can lead to healing unexpectedly deleting some policy
files as dangling due to a mistake in understanding when `isObjectDangling`
is considered to be 'true'.
This issue happened in commit 30135eed86
when we assumed the validMeta with empty ErasureInfo is considered
to be fully dangling. This implementation issue gets exposed when
the server is starting up.
This is most easily seen with multiple-pool setups because of the
disconnected fashion pools that come up. The decision to purge the
object as dangling is taken incorrectly prior to the correct state
being achieved on each pool, when the corresponding drive let's say
returns 'errDiskNotFound', a 'delete' is triggered. At this point,
the 'drive' comes online because this is part of the startup sequence
as drives can come online lazily.
This kind of situation exists because we allow (totalDisks/2) number
of drives to be online when the server is being restarted.
Implementation made an incorrect assumption here leading to policies
getting deleted.
Added tests to capture the implementation requirements.
