Emit warning if ping socket select takes longer

We experienced Zookeeper session timeouts within our applications
with Kazoo because of GIL contention blocking the Kazoo thread and
its ping loop. The thread will now emit a warning in such a case.

kazoo/protocol/connection.py

@@ -589,10 +589,17 @@ def _connect_attempt(self, host, hostip, port, retry):
                     # Ensure our timeout is positive
                     timeout = max([read_timeout / 2.0 - jitter_time,
                                    jitter_time])
+                    select_start_time = time.time()
                     s = self.handler.select([self._socket, self._read_sock],
                                             [], [], timeout)[0]
 
                     if not s:
+                        select_block_time = time.time() - select_start_time
+                        if select_block_time > read_timeout:
+                            self.logger.warning(
+                                "Socket select took longer than expected: %0.5f",
+                                select_block_time
+                            )
                         if self.ping_outstanding.is_set():
                             self.ping_outstanding.clear()
                             raise ConnectionDropped(