Make number of replication workers configurable. (#11379)

MINIO_API_REPLICATION_WORKERS env.var and
`mc admin config set api` allow number of replication
workers to be configurable. Defaults to half the number
of cpus available.

Co-authored-by: Poorna Krishnamoorthy <poorna@minio.io>

cmd/config/errors.go

@@ -281,4 +281,10 @@ Example 1:
 		"",
 		"Refer to https://docs.min.io/docs/minio-kms-quickstart-guide.html for setting up SSE",
 	)
+
+	ErrInvalidReplicationWorkersValue = newErrFn(
+		"Invalid value for replication workers",
+		"",
+		"MINIO_API_REPLICATION_WORKERS: should be > 0",
+	)
 )