Drop some backwards-compatibility code in config.py

Drops some old backwards-compatibility code that was present in
config.py.  Also updates and moves the unit tests into the unit
subdirectory.
Refactor import_class() and find_entrypoint()

There was a lot of duplicated code all over the place because of the
way import_class() and find_entrypoint() were being used.  This
refactors all of that code into find_entrypoint(), reducing the
duplication.  Note that this breaks test_database.py, because that
file is in the process of being rewritten.
Eliminate TurnstileRedis

Eliminate the TurnstileRedis class by promoting the added methods
(limit_update() and command()) to functions.  This also allows the
Redis client class to be set in configuration, which might be
necessary to use some forms of Redis sharding.  Note that
test_database.py is not updated; it is due for a rewrite (see next
commit).