- changed most db accesses to be asynchronous
  (this was way more difficult than anticipated)
- face id allocation is still synchronous (and has to be as long as
  we want to allow multiple instances running in parallel)
- facemap is cached locally, so subsequent accesses are fast
- the alternative would be to store faceids, not tileids, in the
  in-memory map, but then mapcache tiles are non-displayable
  unless the server already sent them.
- tile loading and map cache load/store are fully asynchronous
- replaced 32x32 dialogue and noface images by 64 bit versions
- properly scale special dialogue/noface textures to tilesize