[http] SNI proxying, healthcheck action, populate authorization, better handle nested...

[http] SNI proxying, healthcheck action, populate authorization, better handle nested authorization, docs

- added readme, because this is seriously getting too complicated for me to configure from memory
- refactored connection acceptance with a new `net.Listener` implementation that supports SNI, with virtual listeners that can be used with `http.Server`
- foundations laid for bare-TCP SNI proxying too, but implementation not started yet
- added `healthcheck` action
- added a mutable request context at the logging middleware level, to bubble up request data to the logger
- propagate SAML authorization state to global request context
- SAML action now skips if request was previously authorized
- got multiple listeners + multiple vhosts per listener working

No breaking config changes.