Upper limit to memory size of walker processes. If a walker process
exceeds this limit, it is re-started (at the same point it left off)
by the dispatcher, at most once. If the same child repeatedly exceeds
this limit, the walk may stop until it is re-started via schedule or
manually. Note that this is a soft, not hard limit: the process may
exceed it briefly, yet will exit gracefully and not crash if so.
