Expose the threaded_creation argument through to dogpile.core.
Expose the threaded_creation argument through to dogpile.core.

This relies on https://bitbucket.org/zzzeek/dogpile.core/pull-request/2

wednesdaysunny/dogpile.cache,thruflo/dogpile.cache,thruflo/dogpile.cache,dprince/dogpile.cache,dprince/dogpile.cache