Chorde

Improvements
- Reduced GC pressure by cleaning up callback list on done
futures immediately after popping the callbacks for
execution. This breaks cycles and allows freeing objects
through RC instead of GC, especially since callbacks tend
to be closures referencing a lot of application state.

Bugfixes
- Fixed packaging of pxd files

Added
- Added setdefault and get to exported cython methods
in cuckoocache.pxd so external modules can optimally
interact with it using its dict interface

Improvements
- Improved async latency by executing cleanup tasks
after notifying the results, rather than before.

Bugfixes
- Fixed bug in clients.Base.get that could cause clients that
used the base implementation to return stale results when
invoked with a custom default.
- Fixed coherence OOB updates, which in some circumstances
could renew a stale value instead of fetch a fresh shared
one.
- Fix assorted thread safety issues in the lockless Threadpool
implementation

Changed
- Compatibility with Cython 0.28
- Cythonized decorators and some base clients to lower caching
overhead in the most commonly used patterns
NOTE: cythonized decorators aren't picklable, so client code
may have to be adapted to not depend on the picklability of
decorated functions

Improvements
- Small performance improvements
- Prevent busy looping in async waitkey in some corner cases

Bugfixes
- Throttle coherence tics when trying to refresh stale pending
entries in the broker. Under some conditions (forgotten
tasks and very high timeouts), a very high rate of tic
requests could be generated, causing channel overload.

Changed
- Added some useful debug logging to chorde.mq.coherence

Added
- Add expire() calling mode to decorated functions to perform
an expire on the relevant key.

Bugfixes
- Fix combined use of coherence and renew_ttl. Renew
would take precedence over the refresh operation,
perpetuating stale results