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: Log a message once per minute
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This module sends `-- MARK --` to the log once per minute. This may be
useful to give a sense of how busy the server is or see that logging and
timers are still working.