Set scheduler log sizes automatically based on available memory

[gjoseph92]

There are frequent reports of scheduler memory growing over time:

• Scheduler memory just keep increasing in idle mode #5509
• Are reference cycles a performance problem? #4987 (comment)
• Scheduler memory leak / large worker footprint on simple workload #3898 (there is a different problem here; memory is leaking even with logs turned off, but turning off logs was necessary to debug)
• What's the best way of diagnosing scheduler memory issues? #4998

They often involve memory graphs that look like:

It's very likely that there is a real bug in the scheduler causing memory to accumulate (#3898 (comment)), but often the steep slope on these graphs is caused by various logs on the scheduler accumulating, such as:

• transition_log - distributed.scheduler.transition-log-length
• log - distributed.scheduler.transition-log-length (should maybe be distributed.admin.log-length?)
• events - distributed.scheduler.events-log-length
• computations - distributed.diagnostics.computations.max-history
• Node._deque_handler - distributed.admin.log-length

I propose two things:

1. Log lengths should be set as a percentage of available memory, not as a length—this is much easier for users to configure
   Note that for some/most of these, that may be difficult to do accurately, since the size of the entries is unknown. A rough estimate is probably okay.
2. A memory-cleanup callback that runs, say, once a second, and clears our excess logs if the scheduler is under memory pressure.

[fjetter]

xref #4762 for the various pieces of logging mentioned here