writeBack of db file is supposed to be a short potentially blocking operation inside end of bracket. Use uninterruptibleMask_
Slave.wait: Take no printer. Cancel all slaves does a quicker 2-pass
cancelAllSlaves: Disallow exceptions
Implement -k keepgoing
fs_override: Instead of ASSERT failures upon server death, propagate errors instead of executing libc FS access calls
TODO
Revert "Convert Parallelism cell to be MVar: Fix over-parallelism bug"

This reverts commit ad90ff4167cd1e90337b562ea7887a338e6f26ca.
CellKilled in case of async exceptions. Do not try to re-allocate parallelism expensively when being killed with async exception
Parallelism: Use a state machine to support over-parallel allocations/releases