Speed optimizations, some code cleanup, Python 3.7.x

• Python 3.7+ is now required because some Sqlite3 library changes are only available in that version
  of python or later. These are changes that allow backing up in-memory to on-disk databases and the reverse.
• This adds some optimizations such as adding an index on country and reducing some unique value lengths
  (used for deduplicating similar requests) by subsitituing a hash for a longer string.
• Now loads database from disk into memory on startup and saves to disk again after new items are added.
  • This speeds up data insertion and stat generation speed at the expense of using some memory.
  • If the sqlite database gets too large then storing in memory on normal hardware may not be feasible.
    However, at that scale a number of kinds of re-architecting may be required, even if not storing in
    memory.
• Refactoring of config.py to make it look more like a standard Python object, though it is a singleton.
• Rudimentary tests added.
  • Examples: python3 -m unittest test/test_main.py and python3 -m unittest test/config/test_config.py