go/zodb: Require drivers to close watchq on Close
If we don't require drivers to stop sending to watchq after Close, there could be many deadlock scenarios, for example: - client called drv.Close(); no longer listens to watchq; driver is stuck sending to watchq, or - client called drv.Close(), which itself waits for tasks spawned inside driver to complete, which are stuck sending to watchq because client no longer does <-watchq. The change is similar in spirit to safety guaranty provided by high-level Watcher where after DelWatch call it is guaranteed that there will be no more sends to subscribed watchq (see c41c2907 "go/zodb: High-level watching - initial draft") for details. All drivers don't provide requested guarantee yet. We'll be fixing them one-by-one in followup commits.
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