Currently, span scavenging was done nearly identically in two different
locations. This change deduplicates that into one shared routine.

For #14045.

Change-Id: I15006b2c9af0e70b7a9eae9abb4168d3adca3860
Reviewed-on: https://go-review.googlesource.com/c/139297
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>