This way databases are allowed to be GC'ed when there are no open
connections to them.
As a side effect, databases are no longer kept alive for the duration of
a browsing session. This will be addressed once IndexedDB gets proper
on-disk persistence. For now, avoiding memory leaks is the better
trade-off.
With this change the number of live `Window` objects in GC graph
captured by `test-web -j 1 --dump-gc-graph` goes down from 50 to 25.
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