v6.0.0-beta.4

• Break out of busy loops automatically when the number of synchronous I/O
  operations moves beyond a built-in threshold. This avoids the ZeroMQ
  background I/O process(es) starving the Node.js event loop when it can process
  messages faster than the application. This could have caused decreased
  responsiveness and/or high memory usage. This only happens when
  sending/receiving messages as quickly as possible, such as in a benchmark or
  in test code.

• Fixed a memory leak in socket construction that would manifest itself when
  repeatedly creating many sockets.