This change modifies the behavior of span allocations to no longer
prefer the free treap over the scavenged treap.

While there is an additional cost to allocating out of the scavenged
treap, the current behavior of preferring the unscavenged spans can
lead to unbounded growth of a program's virtual memory footprint.

In small programs (low # of Ps, low resident set size, low allocation
rate) this behavior isn't really apparent and is difficult to
reproduce.

However, in relatively large, long-running programs we see this
unbounded growth in free spans, and an unbounded amount of heap
growths.

It still remains unclear how this policy change actually ends up
increasing the number of heap growths over time, but switching the
policy back to best-fit does indeed solve the problem.

Change-Id: Ibb88d24f9ef6766baaa7f12b411974cc03341e7b
Reviewed-on: https://go-review.googlesource.com/c/148979
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Austin Clements <austin@google.com>