and to deliver to watchq only and all events in (at₀, +∞] range.

This continues 4d2c8b1d (go/zodb: Require drivers to provide
notifications for database change events) and makes initialization
semantics to process invalidations easier for both application-level ZODB
layer, and for low-level users that work with drivers directly:

- there is no possibility that an event will come from watchq with tid <
  current user head.

- there is no possibility that watcher will start delivering events
  after at₀, but not immediately after it, i.e. users can rely that they
  won't loose an event.

This correctness invariants should be easy to provide in drivers.