The current usage of sync.Pool is leaky because it stores an arbitrary
sized buffer into the pool. However, sync.Pool assumes that all items in the
pool are interchangeable from a memory cost perspective. Due to the unbounded
size of a buffer that may be added, it is possible for the pool to eventually
pin arbitrarily large amounts of memory in a live-lock situation.

As a simple fix, we just set a maximum size that we permit back into the pool.

We do not need to fix the use of a sync.Pool in scan.go since the free method
has always enforced a maximum capacity since the first commit of the scan logic.

Fixes #27740
Updates #23199

Change-Id: I875278f7dba42625405df36df3e9b028252ce5e3
Reviewed-on: https://go-review.googlesource.com/136116Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>