Process hang while invoke ConnectionPool.get_connection()

Hi, may be I found a bug in ConnectionPool.get_connection().

You can reproduce the problem through the following steps:

1. Assume the mariadb server is running on host A, and you do follow steps with host B
2. Create a connection pool
   ```
   _pool = mariadb.ConnectionPool(
   host=constant.DB_HOST,
   port=constant.DB_PORT,
   user=constant.DB_USER,
   password=constant.DB_PASSWORD,
   pool_name=constant.DB_POOL_NAME,
   db=constant.DB_NAME,
   pool_size=constant.DB_POOL_SIZE,
   autocommit=True)
   ```
3. Disconnect host B's network.(By shutdown the NIC)
4. Reconnect host B's network
5. invoke `ConnectionPool.get_connection()`

Then the program will fall into a dead cycle which "ping-failed-continue-ping". I think, this is a bug