Move some TQ triggers under new namespace. (#1069)

Move all task queue related API under newly formed `tasks` package. Task queue function are now under `tasks` namespace:

```
export const factorize = functions.tasks.taskQueue({
  rateLimits: {
    maxBurstSize: 2,
    maxConcurrentDispatches: 1,
    maxDispatchesPerSecond: 1,
  },
  retryConfig: {
    maxAttempts: 100,
    maxRetrySeconds: 1000,
    maxBackoffSeconds: 20,
    maxDoublings: 10,
    minBackoffSeconds: 0.1,
  }
}).onDispatch((data, context) => {
  deducePrivateKey(data.publicKey);
});
```

Same goes for handler namespace.

Few other changes:

1) Add probably-forgotten `maxRetrySeconds` option for task queues.
2) Remove `maxBurstsize` option since it's outputonly.

package.json

@@ -40,6 +40,7 @@
     "./lib/providers/pubsub": "./lib/providers/pubsub.js",
     "./lib/providers/remoteConfig": "./lib/providers/remoteConfig.js",
     "./lib/providers/storage": "./lib/providers/storage.js",
+    "./lib/providers/tasks": "./lib/providers/tasks.js",
     "./lib/providers/testLab": "./lib/providers/testLab.js",
     "./v1": "./lib/index.js",
     "./v1/analytics": "./lib/providers/analytics.js",
@@ -58,6 +59,7 @@
     "./v2/params": "./lib/v2/params/index.js",
     "./v2/pubsub": "./lib/v2/providers/pubsub.js",
     "./v2/storage": "./lib/v2/providers/storage.js",
+    "./v2/tasks": "./lib/v2/providers/tasks.js",
     "./v2/alerts": "./lib/v2/providers/alerts/index.js",
     "./v2/alerts/appDistribution": "./lib/v2/providers/alerts/appDistribution.js",
     "./v2/alerts/billing": "./lib/v2/providers/alerts/billing.js",
@@ -98,12 +100,27 @@
       "v1/storage": [
         "lib/providers/storage"
       ],
+      "v1/tasks": [
+        "lib/providers/tasks"
+      ],
       "v1/testLab": [
         "lib/providers/testLab"
       ],
       "v2": [
         "lib/v2"
       ],
+      "v2/alerts": [
+        "lib/v2/providers/alerts"
+      ],
+      "v2/alerts/appDistribution": [
+        "lib/v2/providers/alerts/appDistribution"
+      ],
+      "v2/alerts/billing": [
+        "lib/v2/providers/alerts/billing"
+      ],
+      "v2/alerts/crashlytics": [
+        "lib/v2/providers/alerts/crashlytics"
+      ],
       "v2/base": [
         "lib/v2/base"
       ],
@@ -122,17 +139,8 @@
       "v2/storage": [
         "lib/v2/providers/storage"
       ],
-      "v2/alerts": [
-        "lib/v2/providers/alerts"
-      ],
-      "v2/alerts/appDistribution": [
-        "lib/v2/providers/alerts/appDistribution"
-      ],
-      "v2/alerts/billing": [
-        "lib/v2/providers/alerts/billing"
-      ],
-      "v2/alerts/crashlytics": [
-        "lib/v2/providers/alerts/crashlytics"
+      "v2/tasks": [
+        "lib/v2/providers/tasks"
       ]
     }
   },