Fix locking bug

Implementing mutex locks involved having one lock per slot in the cache. Somewhere between 50k and 100k dataset samples, this fails because Python itself couldn't create a list with that many lock objects.

Solved by removing all mutex locking. This is generally fine -- in each epoch, the same datapoint will never be accessed twice anyway.

If reintroducing in future, there are basically three options:

• lock for each slot (like before), but all locking gets automatically disabled for large datasets.
• one lock for the whole object, but it protects writes only. This would slow down the first epoch, but then be fine.
• Implement both of the above, with a flag for advanced users to toggle the options