This change uses the updated ldap library in minio/pkg (bumped
up to v3). A new config parameter is added for LDAP configuration to
specify extra user attributes to load from the LDAP server and to store
them as additional claims for the user.
A test is added in sts_handlers.go that shows how to access the LDAP
attributes as a claim.
This is in preparation for adding SSH pubkey authentication to MinIO's SFTP
integration.
```
==================
WARNING: DATA RACE
Read at 0x0000082be990 by goroutine 205:
github.com/minio/minio/cmd.setCommonHeaders()
Previous write at 0x0000082be990 by main goroutine:
github.com/minio/minio/cmd.lookupConfigs()
```
Create new code paths for multiple subsystems in the code. This will
make maintaing this easier later.
Also introduce bugLogIf() for errors that should not happen in the first
place.
* creating a byte buffer for SFTP file segments
* Adding an error condition for when there are
remaining segments in the queue
* Simplification of the queue using a map
