Back in Go 1.4, memstats.next_gc was both the heap size at which GC
would trigger, and the size GC kept the heap under. When we switched
to concurrent GC in Go 1.5, we got somewhat confused and made this
variable the trigger heap size, while gcController.heapGoal became the
goal heap size.

memstats.next_gc is exposed to the user via MemStats.NextGC, while
gcController.heapGoal is not. This is unfortunate because 1) the heap
goal is far more useful for diagnostics, and 2) the trigger heap size
is just part of the GC trigger heuristic, which means it wouldn't be
useful to an application even if it tried to use it.

We never noticed this mess because MemStats.NextGC is practically
undocumented. Now that we're trying to document MemStats, it became
clear that this field had diverged from its original usefulness.

Clean up this mess by shuffling things back around so that next_gc is
the goal heap size and the new (unexposed) memstats.gc_trigger field
is the trigger heap size. This eliminates gcController.heapGoal.

Updates #15849.

Change-Id: I2cbbd43b1d78bdf613cb43f53488bd63913189b7
Reviewed-on: https://go-review.googlesource.com/29270
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Rick Hudson <rlh@golang.org>