Manager._state not updated after reconnect

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.

Start by tracing the Manager's handling of the open, close, and reconnect events emitted by MongoClient, focusing on where _state changes. Confirm the reconnect path leaves _state at STATE.OPEN after the event sequence, and add or run a test covering that sequence.

Written by the indexing model from the issue text.