Do not allow a MongoClient to be reused after it is closed

A MongoClient instance can be re-used after being closed, for example this is valid:

client = MongoClient()

client.test.test.find_one()

client.close()

client.test.test.find_one()

This behavior is odd because closing a resource usually means it is no longer needed; a closed file descriptor does not magically reopen itself. No other MongoDB drivers that work this way as far as I know.

This automatic reopening behavior also leads to bugs and implementation complexity, especially while threading is involved. For example some related bugs are:

• PYTHON-1897
• PYTHON-1921
• PYTHON-2092
• PYTHON-2014

For more motivation, while implementing PYTHON-2462 I had to include special logic to circumvent the Pool paused state when the client is closed to avoid breaking the behavior of test.test_threads.TestThreads.test_client_disconnect. We should consider whether we want to remove that logic and instead raise an error on all operations after a client.close(). This would be a breaking change that could go in 4.0.