parallelize startup poll

puts each check for a running spawner in a coroutine and runs them all concurrently.

Note: this will only improve performance when a large number of Spawners are running and `yield spawner.poll()` takes a nontrivial amount of time.
This is because these are coroutines, not threads. If instantiating Spawners themselves takes a long time, performance will not be affected.

jupyterhub/app.py

@@ -1204,8 +1204,6 @@ class JupyterHub(Application):
     def init_spawners(self):
         db = self.db
 
-        user_summaries = ['']
-
         def _user_summary(user):
             parts = ['{0: >8}'.format(user.name)]
             if user.admin:
@@ -1225,10 +1223,8 @@ class JupyterHub(Application):
             yield self.proxy.delete_user(user, server_name)
             yield user.stop(server_name)
 
-        for orm_user in db.query(orm.User):
-            self.users[orm_user.id] = user = User(orm_user, self.tornado_settings)
-            self.log.debug("Loading state for %s from db", user.name)
-            for name, spawner in user.spawners.items():
+        @gen.coroutine
+        def check_spawner(user, name, spawner):
             status = 0
             if spawner.server:
                 try:
@@ -1253,11 +1249,23 @@ class JupyterHub(Application):
                     spawner.server = None
                 else:
                     self.log.debug("%s not running", spawner._log_name)
-            db.commit()
 
-            user_summaries.append(_user_summary(user))
+        # parallelize checks for running Spawners
+        check_futures = []
+        for orm_user in db.query(orm.User):
+            self.users[orm_user.id] = user = User(orm_user, self.tornado_settings)
+            self.log.debug("Loading state for %s from db", user.name)
+            for name, spawner in user.spawners.items():
+                f = check_spawner(user, name, spawner)
+                check_futures.append(f)
 
-        self.log.debug("Loaded users: %s", '\n'.join(user_summaries))
+        # await checks after submitting them all
+        for f in check_futures:
+            yield f
+
+        user_summaries = map(_user_summary, self.users.values())
+
+        self.log.debug("Loaded users:\n%s", '\n'.join(user_summaries))
         db.commit()
 
     def init_oauth(self):