process: add _getActiveHandles(), _getActiveRequests()

process: add _getActiveHandles(), _getActiveRequests()

* process._getActiveHandles() returns a list containing all active handles
  (timers, sockets, etc.) that have not been unref'd.

* process._getActiveRequests() returns a list of active requests (in-flight
  actions like connecting to a remote host, writing data to a socket, etc.).