An installation that holds a key for the TREK services and has no
NOMINATIM_URL of its own now sends the OpenStreetMap search and the
reverse lookups to photon.liketrek.com first. The answers are read into
Nominatim's shape, so the place records stay the same, and every miss
(refused key, allowance spent, timeout, error, foreign body) falls back
to Nominatim, with a one minute pause after four misses in a row.
TREK_PHOTON_URL points at another copy, and the default of
TREK_SERVICES_HOSTS now lists photon.liketrek.com and
valhalla.liketrek.com beside places.liketrek.com.
Maurice