Re: O(n) tasks cause lengthy startups and checkpoints

Here's a new revision where I've attempted to address all the feedback I've
received thus far.  Notably, the custodian now uses a queue for registering
tasks and determining which tasks to execute.  Other changes include
splitting the temporary file functions apart to avoid consecutive boolean
flags, using a timestamp instead of an integer for the staging name for
temporary directories, moving temporary directories to a dedicated
directory so that the custodian doesn't need to scan relation files,
ERROR-ing when something goes wrong when cleaning up temporary files,
executing requested tasks immediately in single-user mode, and more.

-- 
Nathan Bossart
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