close_async
Description:
public virtual async bool close_async (int io_priority = DEFAULT, Cancellable? cancellable = null) throws IOError
  
  Requests an asynchronous closes of the stream, releasing resources related to it.
 When the operation is finished callback will be called. You can then call close_async.end to 
      get the result of the operation.
For behaviour details see close.
The asynchronous methods have a default fallback that uses threads to implement asynchronicity, so they are optional for inheriting classes. However, if you override one you must override all.
Parameters:
| this | A InputStream. | 
| io_priority | the I/O priority of the request | 
| cancellable | optional cancellable object | 
| callback | a TaskReadyCallback to call when the request is satisfied | 
| user_data | the data to pass to callback function |