Drop the MaxMemInCellQueues lower limit down to 256 MB.

on #9686, gmorehose reports that the 500 MB lower limit is too high
for raspberry pi users.

This is a backport of 647248729fa65f0e51d062e2af8f4e8b38592bf5 to 0.2.4.

Note that in 0.2.4, the option is called MaxMemInCellQueues.