Circular dependencies make GC unpredicable #214

Synchronicity currently creates circular references:

async_method [-> Interfaces attribute] -> wrapper_method -> synchronizer instance -> thread + event loop -> [possibly references to the original async_method]
         ^                                     v
          ---------------------<---------------

This leads to unpredictable gc behavior of the synchronizer instance (normally at interpreter shutdown) since the gc normally wants to collect any parent before it collects its attributes/references.

This is what causes the Synchronizer.__del__ to never run at the moment.