use_cassette decorated functions are not reentrant

[colonelpanic8]

This probably isn't a huge issue, but I just realized that CassetteContextDecorator is not threadsafe. If you were to call a function that is decorated with use_cassette from two different threads, the __finish attribute of the function could be overwritten before the first call to the function returns. This would make it so the first unpatch never occurs, and also attempt to do a next() on a None object.

[colonelpanic8]

I think that this is actually a pretty serious issue. If there is a test runner that does single process threaded test execution, decorated functions could present a problem.