Add startAfter and endBefore methods for firebase_database

[Tom3652]

Hello !

I am simply trying to fetch messages from the database using a query such as :

database.reference().child("messages").startAt(createdAt + 1).limitToFirst(30)

However as you can see, i have to do +1 to avoid the "equal" part from the startAt because i already have fetched this value.
The createdAt is the timestamp as time since epoch in millis.

It would be great to have the same methods as native SDK : startAfter and endBefore but it's not a blocking point.