Standalone weed s3 created a master client and registered the receiving
SeaweedS3IamCache gRPC service, but never wrapped its credential store
with the propagating store. Only the filer-embedded path called
SetMasterClient, so IAM mutations on one s3 pod never reached peers; they
served a stale in-memory identity cache and returned InvalidAccessKeyId
until restarted.
Wrap the credential store with the master client when one is available,
mirroring the filer path, so mutations fan out over the existing gRPC
cache service.