Fix PHP-686: Constant reconnect attempts to a server that is down

This is because on every single 'new MongoClient' we would do a full
topology discovery and attempt to connect to every single server we
could find - even though we were previously fully aware of the fact the
server was down.
This may have seemed like a good idea, we would quickly discover when
a server became available again..

However, if the machine is completely off we have to wait until the
connect() times out. Again. And Again. And Again, on every 'new
MongoClient'.
In practice that means all requests coming into the application will
have to wait 1second (default) times down servers, which ofcourse is
unacceptable.