- Rename `ASDisplayNodeAssertLockUnownedByCurrentThread` to `ASDisplayNodeAssertLockNotHeld`, and `ASDisplayNodeAssertLockOwnedByCurrentThread` to `ASDisplayNodeAssertLockHeld` -> shorter and hopefully easier to distinguish between the two.
- Add assertions to `_locked_` and `_u_` (i.e "unlocked") methods.
- Turn `CHECK_LOCKING_SAFETY` flag on by default. After TextureGroup#1022 and TextureGroup#1023, we're in a good shape to actually enforce locked/unlocked requirements of internal methods. Our test suite passed, and we'll test more at Pinterest after the sync this week.
- Fix ASVideoNode to avoid calling `play` while holding the lock. That method inserts a subnode and must be called lock free.
- Other minor changes.

…huy/Texture into HN-Stricter-Locking-Safety-Checks

…layer backed, the layer should always be set if loaded