Fixes bug and improves store list refresh upon sync.

When the logs from another device are updated on disk, the MOC needs to
flush the cache for that database. Now using a “blink” approach where
the URL of the store is set again, instead of a “swap” approach where
the persistent store is removed and a new one is created. There was
also a bug where the list of read-only databases (persistent stores)
would be continuously growing.
Triggers a "blink" of the read-write log database of the local device after saving to disk.

The “blink” is necessary for Dropbox to upload the modified database to
their servers. The “blinking” is done by simply setting again the store
URL to the same value as before. It is not needed on iOS where the
files are  under the control of the app and will need to be manually
uploaded to online file services.