Shut down the eel server less aggressively

At the moment, every time a websocket connection closes, we check 1
second later to see if there any any connections open. If a user opens
and terminates multiple websocket connections, this can mean that one of
the earlier "1 second laters" comes along during a later
shutdown/reconnect and closes the server, despite the user navigating
through the app in a reasonable way.

This patch moves this check to a "spawn later" gevent greenlet that can
be terminated and rescheduled. Every time a websocket is closed, we
cancel any previously-running check and schedule a new one for 1 second
in the future, which should mean that Eel shuts down less often during
valid navigation step/s.

Fixes #248

eel/__init__.py

@@ -1,6 +1,7 @@
 from builtins import range
 from io import open
 
+from gevent.threading import Timer
 import gevent as gvt
 import json as jsn
 import bottle as btl
@@ -25,6 +26,7 @@
 _js_functions = []
 _mock_queue = []
 _mock_queue_done = set()
+_shutdown = None
 
 # The maximum time (in milliseconds) that Python will try to retrieve a return value for functions executing in JS
 # Can be overridden through `eel.init` with the kwarg `js_result_timeout` (default: 10000)
@@ -326,17 +328,24 @@ def _expose(name, function):
     _exposed_functions[name] = function
 
 
+def _detect_shutdown():
+    if len(_websockets) == 0:
+        sys.exit()
+
+
 def _websocket_close(page):
+    global _shutdown
+
     close_callback = _start_args.get('close_callback')
 
     if close_callback is not None:
         sockets = [p for _, p in _websockets]
         close_callback(page, sockets)
     else:
-        # Default behaviour - wait 1s, then quit if all sockets are closed
-        sleep(1.0)
-        if len(_websockets) == 0:
-            sys.exit()
+        if _shutdown:
+            _shutdown.kill()
+
+        _shutdown = gvt.spawn_later(1.0, _detect_shutdown)
 
 
 def _set_response_headers(response):