A caching API built around the concept of a "dogpile lock", which allows
continued access to an expiring data value while a single thread generates
a new value.

dogpile.cache builds on the dogpile.core locking system, which implements the
idea of "allow one creator to write while others read" in the abstract.
Overall, dogpile.cache is intended as a replacement to the Beaker caching
system, the internals of which are written by the same author.
