Close Pipes used in FBProcessStream
Close Pipes used in FBProcessStream

Summary:
Without calling `closeFile` on `fileHandleForWriting` we will leak pipes. From the documentation of `NSPipe`:

> You use the returned file handle to write to the pipe using NSFileHandle's writeData: method. When you are finished writing data to this object, send it a closeFile message to delete the descriptor. Deleting the descriptor causes the reading process to receive an end-of-data signal (an empty NSData object).

We have the opportunity to do this in the lifecycle events here as all usage of `NSPipe` is now localized to `FBProcessStream`

Differential Revision: D7083552

fbshipit-source-id: 8691d5cb18a93365cf6825eb646c8c72de3daa70

facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl