when a transaction is about to be written. It would be bad if we
"flipped" the cache twice during a transaction, as we would have
incomplete transaction data.  Note that this means that we could
exceed the target cache size if the data for a transaction exceeded
the target size (/2).

Changed the way that the storage is closed. Our close method may be
called from a separate thread than the async main loop. We only want
the async main loop to close our _call (rpc) object. Instead of
closing it directly, we call closeIntensionally(), which, in return,
requests that the async main loop closes it, via the select trigger.