Verify that the watch API emits one event per atomic transaction at the datastore layer, and then document that behavior in the API

[jakedt]

Problem Statement

Right now, it's ambiguous how many events might get batched up in a single WatchResponse message, due to the naming of the resume zedtoken of changes_through.

Solution Brainstorm

In actuality, what should happen, and be enforced across all datastores, is that a single WatchResponse message contains mutations from only a single atomic datastore transaction, and our API docs should be updated to guarantee that.