kvclient: ship entire write buffer on leaf transcations

[arulajmani]

In service of #72614

Once we start buffering writes on the client, a leaf transaction that attempts to read-its-own-writes will need access to the buffer as well. As a first step, to ensure correctness, we should ship the entire write buffer on every leaf transaction. In the future, we may then be able to constrain the buffer shipped on leaf transactions to just the span over which the leaf transaction is created.