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: Extra verbose stanza logging
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Summary
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This module logs the full stanzas that are sent and received into debug
logs, for debugging purposes.