make fsop timeout injectable

Summary:
We have a weak build machine for React Native app: a poor Mac Book Air with HDD, which is very slow.

So, fs operations sometimes fail because of timeout.

We likely don't care about build time, but it's pretty annoying to restart builds sometimes.

In this PR I make timeout injectable via environment variable.

__USAGE__:
export this variable into environment. It's measured in miliseconds.
Example, for 1 minute timeout:
`export REACT_NATIVE_FSOP_TIMEOUT=60000`

If you don't specify it, it will remain default value: `15000`

This should not break anything, but repair slow builds.

Related to : facebook#9373 , facebook#8794 (I think so)

This PR should handle case of issue and close facebook#9373
Closes facebook#9374

Differential Revision: D3709325

Pulled By: bestander

fbshipit-source-id: b00c89e10d05362314546faea7a4524f3d327c97

Author: ColCh

packager/react-packager/src/AssetServer/index.js

@@ -28,9 +28,11 @@ function timeoutableDenodeify(fsFunc, timeout) {
   };
 }
 
-const stat = timeoutableDenodeify(fs.stat, 15000);
-const readDir = timeoutableDenodeify(fs.readdir, 15000);
-const readFile = timeoutableDenodeify(fs.readFile, 15000);
+const FS_OP_TIMEOUT = parseInt(process.env.REACT_NATIVE_FSOP_TIMEOUT, 10) || 15000;
+
+const stat = timeoutableDenodeify(fs.stat, FS_OP_TIMEOUT);
+const readDir = timeoutableDenodeify(fs.readdir, FS_OP_TIMEOUT);
+const readFile = timeoutableDenodeify(fs.readFile, FS_OP_TIMEOUT);
 
 const validateOpts = declareOpts({
   projectRoots: {