mod_pubsub_serverinfo: implemented all basic features This commit replaces the earlier proof-of-concept to a solution that: - reports on remotely-connected domains - uses disco/info to detect if those domains opt-in - publishes domain names for remote domains that do so - caches the disco/info response

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summary: Storage benchmark tool
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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.