Never wait for the cache lock in __traverse__

[Malkiz223]

Closes #84. Split out of #85, as requested - the __traverse__ part alone.

__traverse__ now tries the lock and skips the pass when it is busy: the thread holding the lock may be running Python code (a value's __del__, a key's __eq__, getsizeof), and a collection landing there deadlocked the whole process. Skipping is safe - the cache and its contents just stay reachable until the next pass, and a busy lock means some thread is inside a cache operation and holds a reference to the cache anyway.

All four scripts from #84 run to the end with this change. Two regression tests, for all seven cache types, in a child process with a timeout: a dropped value whose __del__ triggers a full collection, and a key whose __eq__ triggers one in the middle of a probe. On main both fail with "never returned".