Use mutex library instead of repeatedly chaining promises

Summary:
I noticed that after a long-running flipper session, 11% of the memory was composed of millions of tiny PromiseReaction records (https://github.com/domenic/promises-unwrapping#promisereaction-records).

It seems this method of chaining promises meant that it was forming a linked list of these from the most recent, back to the very first invocation.

Instead of using a home-rolled lock, I'm replacing it with an actual library. Haven't verified the fix, but this seems like a better approach regardless.

Reviewed By: passy

Differential Revision: D15853570

fbshipit-source-id: 1626d9f25cba809794b13ee920bdec2cd1d4b874

flow-typed/npm/async-mutex_vx.x.x.js

@@ -0,0 +1,46 @@
+// flow-typed signature: 3f26bdb86dbdfe57e7bf6c11d85e98e4
+// flow-typed version: <<STUB>>/async-mutex_v0.1.3/flow_v0.100.0
+
+/**
+ * This is an autogenerated libdef stub for:
+ *
+ *   'async-mutex'
+ *
+ * Fill this stub out by replacing all the `any` types.
+ *
+ * Once filled out, we encourage you to share your work with the
+ * community by sending a pull request to:
+ * https://github.com/flowtype/flow-typed
+ */
+
+declare module 'async-mutex' {
+  declare module.exports: any;
+}
+
+/**
+ * We include stubs for each file inside this npm package in case you need to
+ * require those files directly. Feel free to delete any files that aren't
+ * needed.
+ */
+declare module 'async-mutex/lib/index' {
+  declare module.exports: any;
+}
+
+declare module 'async-mutex/lib/Mutex' {
+  declare module.exports: any;
+}
+
+declare module 'async-mutex/lib/MutexInterface' {
+  declare module.exports: any;
+}
+
+// Filename aliases
+declare module 'async-mutex/lib/index.js' {
+  declare module.exports: $Exports<'async-mutex/lib/index'>;
+}
+declare module 'async-mutex/lib/Mutex.js' {
+  declare module.exports: $Exports<'async-mutex/lib/Mutex'>;
+}
+declare module 'async-mutex/lib/MutexInterface.js' {
+  declare module.exports: $Exports<'async-mutex/lib/MutexInterface'>;
+}