An earlier performance change, https://golang.org/cl/37406,
made headerFieldTable lookups O(1). The entries in the maps used to perform
these lookups were not evicted along with the original field elements, however
causing a gradual memory leak. This is most apparent when the headers have
highly variable content such as request IDs or timings.

Fixes golang/go#19756

Change-Id: Icdb51527eb671925216350ada15f2a1336ea3158
Reviewed-on: https://go-review.googlesource.com/38781Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>