Make KingSubsite part of ServConf, handle 404 case more gracefully, make
slog endpoint send SSE headers immediately.
Remaining work mostly revolves around the slog endpoint's slog queue. It
builds up even if nobody is listening, and only the first to pull from
the queue gets to handle/emit the slog event.
I noticed that the king's text log file kept filling up, and it's
mostly 'sending chunk "\n"' from eyre. This changes the log level
of every partial send over an active eyre channel to info, instead
of always.
