Support $currentDate field update operator

[AlekSi]

Dot notation is not in scope. Use UnimplementedDot if available.

In a separate PR, timestamp:

"Timestamp": {
        id:     "double",
	update: bson.D{{"$currentDate", bson.D{{"value", bson.D{{"$type", "timestamp"}}}}}},
	stat: &mongo.UpdateResult{
		MatchedCount:  1,
		ModifiedCount: 1,
		UpsertedCount: 0,
	},
        result: bson.D{{"_id", "double"}, {"value", nowTimestamp}},
},

where

nowTimestamp := primitive.Timestamp{T: uint32(time.Now().UTC().Unix()), I: uint32(0)}