Manager._state not updated after reconnect

[ClintHaerinck]

When following events are emitted:

• open
• close
• reconnect
  The connection _state parameter is still STATE.CLOSED from the close event.

This because when a reconnect event is emitted by the MongoClient, the Manager _state parameter isn't updated to STATE.OPEN.

[mathieudutour]

Ah good catch! Want to send a PR to fix it?

[ClintHaerinck]

Here you are. But I don't know if it will be used for long, I read stuff about unified topology. When I enable the 'useUnifiedTopology' option, the events aren't working anymore.