Scheduled update blocks transactions

[diredev]

Podcache will perform a full update of all feeds once a day (and possibly on startup). During the update (which could take a while on slower connections), HSQLDB will lock the table (for update).

The change from c5ae073 means that existing feeds can be read during this time at least, but adding or modifying a feed still has to wait for the entire update operation to complete.

It also seems like HSQLDB does not support lock timeouts (AFAIK), so the transactions literally wait forever.

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Consider improving this situation. Maybe there is a better way (Java-side locks?) to enforce consistency?