This completes __Pyx_SET_REFCNT and __Pyx_SET_SIZE.
See https://bugs.python.org/issue39573

This also remove the associated mutex from the CyLock class.

This commit introduces the following changes:
- 'locked' becomes 'lock'
- 'locked&' becomes 'locked'
- references reachable from a locked reference are noted as 'lock->'