refactor: remove usage of async module (#702)

Author: AVaksman

package.json

@@ -54,7 +54,6 @@
     "@google-cloud/paginator": "^1.0.0",
     "@google-cloud/promisify": "^1.0.0",
     "arrify": "^2.0.0",
-    "async": "^2.0.1",
     "compressible": "^2.0.12",
     "concat-stream": "^2.0.0",
     "date-and-time": "^0.6.3",
@@ -107,6 +106,7 @@
     "node-fetch": "^2.2.0",
     "normalize-newline": "^3.0.0",
     "nyc": "^14.0.0",
+    "p-limit": "^2.2.0",
     "power-assert": "^1.4.4",
     "prettier": "^1.7.0",
     "proxyquire": "^2.0.0",

src/bucket.ts

@@ -1099,7 +1099,7 @@ class Bucket extends ServiceObject {
   combine(
     sources: string[] | File[],
     destination: string | File,
-    options: CombineOptions
+    options?: CombineOptions
   ): Promise<CombineResponse>;
   combine(
     sources: string[] | File[],
@@ -2773,7 +2773,7 @@ class Bucket extends ServiceObject {
 
   setStorageClass(
     storageClass: string,
-    options: SetBucketStorageClassOptions
+    options?: SetBucketStorageClassOptions
   ): Promise<SetBucketMetadataResponse>;
   setStorageClass(
     storageClass: string,
@@ -2827,7 +2827,7 @@ class Bucket extends ServiceObject {
    */
   setStorageClass(
     storageClass: string,
-    optionsOrCallback:
+    optionsOrCallback?:
       | SetBucketStorageClassOptions
       | SetBucketStorageClassCallback,
     callback?: SetBucketStorageClassCallback