Producer thread crashes if client hits fd limit

[wbarnha]

We should handle this a bit more gracefully.

Uncaught error in kafka producer I/O thread
Traceback (most recent call last):
  File "venv/lib/python3.5/site-packages/kafka/producer/sender.py", line 55, in run
    self.run_once()
  File "venv/lib/python3.5/site-packages/kafka/producer/sender.py", line 145, in run_once
    self._client.poll(poll_timeout_ms, sleep=True)
  File "venv/lib/python3.5/site-packages/kafka/client_async.py", line 407, in poll
    metadata_timeout_ms = self._maybe_refresh_metadata()
  File "venv/lib/python3.5/site-packages/kafka/client_async.py", line 631, in _maybe_refresh
_metadata
    self._maybe_connect(node_id)
  File "venv/lib/python3.5/site-packages/kafka/client_async.py", line 247, in _maybe_connect
    conn.connect()
  File "venv/lib/python3.5/site-packages/kafka/conn.py", line 140, in connect
    self._sock = socket.socket(self.afi, socket.SOCK_STREAM)
  File "/usr/local/lib/python3.5/socket.py", line 134, in __init__
    _socket.socket.__init__(self, family, type, proto, fileno)
OSError: [Errno 24] Too many open files