JdbcBinaryStore's expiration locks buckets indefinitely

The buckets are locked in eviction thread (in the main purge method), while unlocked in BucketPurger.call() which is executed in persistence thread. The unlock fails and the buckets stay locked indefinitely.

Another error is that the Bucket class is not serializable.

This is also a bug in BaseStoreTest as this uses WithinThreadExecutor as the executor for purging while usually this is done in different thread. Moreover, as the purge method is actually not obliged to purge anything, the test does not test the purging itself, but rather a check for expired entry when it is loaded (contains operation). The purging should be enforced by purge listener (calling the purge method until all entries are purged).