with missing nextMarker with delimiter based listing,
top level prefixes beyond 4500 or max-keys value
wouldn't be sent back for client to ask for the next
batch.
reproduced at a customer deployment, create prefixes
as shown below
```
for year in $(seq 2017 2020)
do
for month in {01..12}
do for day in {01..31}
do
mc -q cp file myminio/testbucket/dir/day_id=$year-$month-$day/;
done
done
done
```
Then perform
```
aws s3api --profile minio --endpoint-url http://localhost:9000 list-objects \
--bucket testbucket --prefix dir/ --delimiter / --max-keys 1000
```
You shall see missing NextMarker, this would disallow listing beyond max-keys
requested and also disallow beyond 4500 (maxKeyObjectList) prefixes being listed
because client wouldn't know the NextMarker available.
This PR addresses this situation properly by making the implementation
more spec compatible. i.e NextMarker in-fact can be either an object, a prefix
with delimiter depending on the input operation.
This issue was introduced after the list caching changes and has been present
for a while.
e5d378931d