Preload schema_resource in class instance variable

* Preloading it in a class instance variable, reduces the
  calls to the file load (500 KB) to one time at class
  load time. This is still faster than a memoizing (even
  in a class function).

* In Rails, the test and production environment pre-load the
  classes, so test and production see only a 1 time hit of
  250 ms at load time.

* In the future, better solutions may be:
1)  C extension (or FFI) with the data staticly stored
2)  using Java/JRuby
3)  storing this data in an external "database"
    (Redis, Postgresql, Elasticsearch, or a self-written deamon)

There is a good chance option (3) will be required for
live data as well, so we may re-use that implementation
for the static data.