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
