option to drop module/function from key_generator functions in util.py

Migrated issue, originally created by jvanasco (jvanasco)

I tracked down a performance issue to usage of dogpile.cache.region:Region.cache_on_arguments in a few places.

The default key generation is provided key generator functions in dogpile.cache.util, with this relevant code shared by all:

    if namespace is None:
        namespace = '%s:%s' % (fn.__module__, fn.__name__)
    else:
        namespace = '%s:%s|%s' % (fn.__module__, fn.__name__, namespace)

My issue was the "base" of the key('%s:%s' % (fn.__module__, fn.__name__)) was incredibly long in a handful of the most-used places, and the keys themselves were taking up a significant chunk of the memory allocated to Redis. Dropping the key length reclaimed a lot of space.

While a key_mangler would solve this, I needed to keep the keys unmangled. The easiest solution was to just reimplement the stock key generator to just use the namespace argument as the key.

This seems like a relatively useful approach for many people, so I wanted to suggest porting it upstream:

I think the relevant changes would be:

1. The key_generators have a new kwarg to omit the module+name prefix (like to_str)
2. cache_on_arguments(_multi) have a new kwarg for the same, which is just passed to the key_generator (again, like to_str)

Start in util.py with the shared key generator functions, then trace cache_on_arguments and cache_on_arguments_multi to see how their keyword arguments reach the generators. Verify that the new option is accepted and forwarded consistently, and that generated keys can omit the module-and-function prefix while preserving the namespace behavior.