refactor(mlcache) use weak table to store LRU instances
refactor(mlcache) use weak table to store LRU instances

The new implementation uses the built-in weak table that is equivalent
to the previous ctype-based system, but without the `__gc` hook that
ended up not being used.

From #48

Signed-off-by: Thibault Charbonnier <3c2e9133e272cb489e2fea0c4328cf56b08e4226@me.com>

thibaultcha/lua-resty-mlcache