Implement heal-upload admin API (#3914)

This API is meant for administrative tools like mc-admin to heal an
ongoing multipart upload on a Minio server.  N B This set of admin
APIs apply only for Minio servers.

`github.com/minio/minio/pkg/madmin` provides a go SDK for this (and
other admin) operations.  Specifically,

  func HealUpload(bucket, object, uploadID string, dryRun bool) error

Sample admin API request:
POST
/?heal&bucket=mybucket&object=myobject&upload-id=myuploadID&dry-run
- Header(s): ["x-minio-operation"] = "upload"

Notes:
- bucket, object and upload-id are mandatory query parameters
- if dry-run is set, API returns success if all parameters passed are
  valid.

cmd/admin-router.go

@@ -64,6 +64,8 @@ func registerAdminRouter(mux *router.Router) {
 	adminRouter.Methods("POST").Queries("heal", "").Headers(minioAdminOpHeader, "object").HandlerFunc(adminAPI.HealObjectHandler)
 	// Heal Format.
 	adminRouter.Methods("POST").Queries("heal", "").Headers(minioAdminOpHeader, "format").HandlerFunc(adminAPI.HealFormatHandler)
+	// Heal Uploads.
+	adminRouter.Methods("POST").Queries("heal", "").Headers(minioAdminOpHeader, "upload").HandlerFunc(adminAPI.HealUploadHandler)
 
 	/// Config operations