Trap exceptions during shutdown of comm port, fix process proxy leak

PR jupyter-server#279 introduced some fixes to address file descriptor leaks - one of
which was to shutdown the communication port.  Since the kernel launcher
listening on the other side may have already terminated, the shutdown
method could throw an exception.  This change catches, logs, then ignores
such exceptions.

While looking into other leaks (in this case memory), it was discovered
that the process proxy instance was being leaked across kernel cycles.
This change addresses that particular leak.

Note: Other PRs have also been submitted to address leaks in
`jupyter_client` and `notebook`.  These are:

[PR 360 - Fix memory leak of kernel Popen object](jupyter/jupyter_client#360)
[PR 361 - Fix memory leak of IOLoopKernelManager object](jupyter/jupyter_client#361)
[PR 3424 - Fix memory leak of iopub object in activity monitoring](jupyter/notebook#3424)

enterprise_gateway/services/kernels/remotemanager.py

@@ -169,6 +169,7 @@ def cleanup(self, connection_file=True):
         # which then prevents process proxy cleanup.
         if self.process_proxy:
             self.process_proxy.cleanup()
+            self.process_proxy = None
         return super(RemoteKernelManager, self).cleanup(connection_file)
 
     def get_connection_info(self, session=False):

enterprise_gateway/services/processproxies/processproxy.py

@@ -26,7 +26,6 @@
 from Crypto.Cipher import AES
 
 
-
 # Default logging level of paramiko produces too much noise - raise to warning only.
 logging.getLogger('paramiko').setLevel(os.getenv('EG_SSH_LOG_LEVEL', logging.WARNING))
 
@@ -701,7 +700,12 @@ def shutdown_listener(self):
                                  "(using remote kill): {}".format(self.comm_ip, self.comm_port,
                                                                   self.kernel_id, str(e)))
             finally:
-                sock.shutdown(SHUT_WR)
+                try:
+                    sock.shutdown(SHUT_WR)
+                except Exception as e2:
+                    self.log.warning("Exception occurred attempting to shutdown communication socket to {}:{} "
+                                     "for KernelID '{}' (ignored): {}".format(self.comm_ip, self.comm_port,
+                                                                      self.kernel_id, str(e2)))
                 sock.close()
 
             # Also terminate the tunnel process for the communication port - if in play.  Failure to terminate