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()
```
* Reduce allocations
* Add stringsHasPrefixFold which can compare string prefixes, while ignoring case and not allocating.
* Reuse all msgp.Readers
* Reuse metadata buffers when not reading data.
* Make type safe. Make buffer 4K instead of 8.
* Unslice
