[local-sync] Make the sync loop interruptible

Summary:
This commit introduces interruptible sync operations. Now, the `SyncWorker`, `FetchFolderList` operation and `FetchMessagesInFolder` operation can be interrupted at several p

To achieve this, this commit adds an Interruptible abstraction, which is an object that can run functions and interrupt them at points marked by the function. For more info o

This commit also splits up the SyncWorker a little bit to make it smaller, byadding a SyncbackTaskWorker.

Test Plan: manual

Reviewers: evan

Reviewed By: evan

Differential Revision: https://phab.nylas.com/D3613

package.json

@@ -17,6 +17,7 @@
     "analytics-node": "2.x.x",
     "async": "^0.9",
     "babel-core": "6.20.0",
+    "babel-plugin-transform-async-generator-functions": "6.17.0",
     "babel-preset-electron": "^0.37.8",
     "babel-preset-react": "6.16.0",
     "babel-regenerator-runtime": "6.5.0",

src/K2

@@ -1 +1 @@
-Subproject commit 45331f71808b223af71c04e4b4407863b327286a
+Subproject commit 2bb760e95db2984edbc89810ae85abd845d8e72d

static/babelrc.json

@@ -3,5 +3,8 @@
     "electron",
     "react"
   ],
+  "plugins": [
+    "transform-async-generator-functions"
+  ],
   "sourceMaps": "inline"
 }