Add support for watch operations on collections.

We should support the original watch API in a similar way to how we ported the MongoClient API's (i.e. replacing Task with RealmAsyncResultTask).

The original stitch API's are described here:
http://stitch-sdks.s3-website-us-east-1.amazonaws.com/stitch-sdks/java/4/com/mongodb/stitch/android/services/mongodb/remote/RemoteMongoCollection.html

watch()
watch​(BsonValue... ids)
watch​(ObjectId... ids)
watchWithFilter​(BsonDocument matchFilter)
watchWithFilter​(Document matchFilter)

From a quick glance, it looks like we need to port the following classes:

• ChangeStream<EventT extends BaseChangeEvent>
• AsyncChangeStream<DocumentT,​ChangeEventT extends BaseChangeEvent<DocumentT>>
• BaseChangeEvent<DocumentT>
• ChangeEvent<DocumentT>
• BaseChangeEventListener<DocumentT,​ChangeEventT extends BaseChangeEvent<DocumentT>>
• ExceptionListener