mod_s2s_keepalive: Isolate source host of pings The incoming_s2s table is not restricted to the current virtualhost so this prevents opening more connections than what's needed. Also prevents useless double sending of one whitespace per local host.

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summary: Allow server administrators to probe any user
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This module lets server administrators send `<presence type="probe"/>`
to any local user and receive their presence in response, bypassing
roster checks.