[ASRangeController] We're already on main thread, remove blocks
Make data source read-only, clarify what's asynchronous
[ASDataController] Clean up some interfaces
[ASDataController] A little more cleanup
[ASDataController] Cleanup
[ASDataController] Restore some changes, exit more often
[ASDataController] Use item counts that we already have rather than requerying them
[ASDataController] Revert weakifications
[ASDataController] Add a mechanism to measure how much work we avoided