Use description package and share selectors

Use description.WriteSelector for write selectors.
Use the Database's read and write selectors when creating a Collection.

GODRIVER-331
GODRIVER-330

Change-Id: I8b5ef75f265840f34a2c7bdedc9636eaac571e94

Author: skriptble

core/topology/server.go

@@ -23,8 +23,7 @@ const minHeartbeatInterval = 500 * time.Millisecond
 var ErrServerClosed = errors.New("server is closed")
 
 // SelectedServer represents a specific server that was selected during server selection.
-// It contains the kind of the typology it was selected from and the read preference that
-// was given during server selection.
+// It contains the kind of the typology it was selected from.
 type SelectedServer struct {
 	*Server
 

mongo/collection.go

@@ -41,15 +41,10 @@ func newCollection(db *Database, name string) *Collection {
 		readPreference: db.readPreference,
 		readConcern:    db.readConcern,
 		writeConcern:   db.writeConcern,
+		readSelector:   db.readSelector,
+		writeSelector:  db.writeSelector,
 	}
 
-	coll.readSelector = description.CompositeSelector([]description.ServerSelector{
-		description.ReadPrefSelector(coll.readPreference),
-		description.LatencySelector(db.client.localThreshold),
-	})
-
-	coll.writeSelector = description.ReadPrefSelector(readpref.Primary())
-
 	return coll
 }
 

mongo/database.go

@@ -43,7 +43,7 @@ func newDatabase(client *Client, name string) *Database {
 		description.LatencySelector(db.client.localThreshold),
 	})
 
-	db.writeSelector = description.ReadPrefSelector(readpref.Primary())
+	db.writeSelector = description.WriteSelector()
 
 	return db
 }