"Reverse Viewport" support in the Deephaven server and Java client

[rcaudy]

As a user, I would like to be able to efficiently subscribe to a range of rows at the end of an updating table. Currently, this requires a 3-way dance:

1. Server notifies client that the table's size has changed
2. Client asks the server to move the viewport to the "new" end
3. Server computes and sends a new effective viewport and snapshot to the client

This introduced extra message traffic and latency for viewport changes.

Alternatively, with a "reverse viewport" subscription, the server will know that the client wants to stay subscribed to rows starting from the end of the table as position 0, and counting up towards the first row of the table. This will allow the server to simply skip straight to step 3, advising the client of its new effective viewport and snapshot, with no explicit request from the client.

We have already added the necessary request message support in Barrage 0.4.0. This ticket is to cover the implementation on the server side and in the Java reference client.