Split gis-data into data and http parts

Split gis-data into data and http parts

gis-data:
  Generic things such as the callback types and cache types.

gis-http:
  HTTP related caching functions (what was GisData)

  This was almost entirely refactored to be single threaded. It also
  uses a GisHttp structure to store data between calls to the fetch
  function. Plugins, etc should call the fetch function from a thread to
  avoid blocking the UI.