waitForFreeConnection() is blocked for condition variable here. To work properly, the _cv should be notified each time the `_checkedOut` is decremented.

1. In one code path, in PoolForHost::done() `_checkedOut` is properly followed by notify, because the only call site is DBConnectionPool::release().

Even though this is correct, it is over-optimization. It is clearer to notify near the place where condition is changed. If this is regarded as an important optimization, a comment should be added.

2. However in DBConnectionPool::decrementEgress() the `_checkedOut` is decremented and the _cv is not notified. The call site is ScopedDbConnection::kill(). This must be fixed.

When using ScopedDbConnection, its deletion will not trigger reusing the connection.

A possible deadlock created by this bug may hit the ReplicaSetMonitor. In Refresher::_refreshUntilMatches() we are using the ScopedDbConnection here. If ReplicaSetMonitor is releasing a scoped connection, this will not trigger anyone waiting for it. If that was the moment RSM detected a change, the deadlock may prevent other blocked threads to take advantage of freed connection.

This problem may not be present in v4.2 and late4r because of significant refactoring like SERVER-35688 that were impossible to backport.