Fix wait_for_schema_agreement deadlock

Fixes #168

Fix works by extracting part of on_down that marks host as down
out of the executor - so it does not need to wait for free thread.
When host is marked as down, wait_for_schema_agreement can finish,
which in turn enables rest of on_down (the part that still runs on
executor) to be executed.

cassandra/cluster.py

@@ -2003,6 +2003,17 @@ def _start_reconnector(self, host, is_host_addition):
         reconnector.start()
 
     @run_in_executor
+    def on_down_potentially_blocking(self, host, is_host_addition):
+        self.profile_manager.on_down(host)
+        self.control_connection.on_down(host)
+        for session in tuple(self.sessions):
+            session.on_down(host)
+
+        for listener in self.listeners:
+            listener.on_down(host)
+
+        self._start_reconnector(host, is_host_addition)
+
     def on_down(self, host, is_host_addition, expect_host_to_be_down=False):
         """
         Intended for internal use only.
@@ -2028,18 +2039,9 @@ def on_down(self, host, is_host_addition, expect_host_to_be_down=False):
             host.set_down()
             if (not was_up and not expect_host_to_be_down) or host.is_currently_reconnecting():
                 return
-
         log.warning("Host %s has been marked down", host)
 
-        self.profile_manager.on_down(host)
-        self.control_connection.on_down(host)
-        for session in tuple(self.sessions):
-            session.on_down(host)
-
-        for listener in self.listeners:
-            listener.on_down(host)
-
-        self._start_reconnector(host, is_host_addition)
+        self.on_down_potentially_blocking(host, is_host_addition)
 
     def on_add(self, host, refresh_nodes=True):
         if self.is_shutdown: