Purge the roster from RAM when the user logs off.
Also.. the `download` flag was being checked for, but never set, causing the
roster to be fetched even if it was cached.
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summary: Storage benchmark tool
...
To benchmark `mod_storage_internal`:
prosodyctl mod_benchmark_storage internal
Does a number (aprox. 10000) of reads and writes and to the selected
storage driver and tells you how long it took.