Mark server unknown and clear the pool when background connections fail.
Eagerly evict threads from the wait queue when pool is paused. Evicted
threads will raise the following error:
AutoReconnect('localhost:27017: connection pool paused')
Introduce PoolClearedEvent and ConnectionPoolListener.pool_ready.
CMAP spec test changes:
- CMAP unit tests should not use real monitors
- Assert that CMAP threads complete all scheduled operations
Added example event loggers and documentation for them to make it easier for users to start using listeners in their code.
JIRA ticket: https://jira.mongodb.org/browse/PYTHON-2076
