Nominatim: limit requests to one per second

The Nominatim usage policy allows "an absolute maximum of 1 request per
second", but nothing throttled submits: holding Enter or clicking the search
button repeatedly sent one request per keypress.

Queue geocode() and reverse() behind a per-instance promise chain that spaces
the start of consecutive requests at least a second apart. Requests are
delayed rather than dropped, and the chain resolves to a timestamp so a failed
request cannot stall the ones behind it.

https://operations.osmfoundation.org/policies/nominatim/