Release note: None

Before this patch we were taking a heap profile once RSS got above 85%
of system memory and then we wouldn't take another profile until we go
below it and then again above it. The problem with the RSS is that it
includes "goIdle" memory (memory allocated from the OS that's not
actually in use) ; I've seen it be up to several gigs.
With this patch, we now take a profile every time a new high-water mark
is reached. The recorded high-water mark is reset once an hour, this way
ensuring that one big measurement doesn't stop us forever from taking
another profile. It's simpler and it has a chance of catching some
further heap increases.

The rationale behind the patch is that we want profiles when the heap is
large more than when the RSS is large. I'm looking at a case where we
took a heap profile when the heap was 4.5 gigs with 2 gigs idle and then
never took one again because of how the heuristic works. And then we
OOMed when the heap was larger and the idle space was lower, but the RSS
was about the same. With this patch, we would have taken a profile at a
more interesting time.

Release note: None