Optimize gzip compression of responses (#15416)

It looks like maintainers aren't very active on the repo
and nytimes/gziphandler#107 has come to a standstill.

I don't want to wait forever for this to go in. Let's use a replace directive
to point to the fork. When the PR gets merged, we can just bump the dependency
and remove the replace directive.

Re: the PR, copying the text from the PR description

This gives 40-50% improvements in CPU with very minor increase
in memory, just as a drop-in replacement. I think that is a very reasonable tradeoff.

The library is mature and safe to be used in production.

```
name                 old time/op    new time/op    delta
GzipHandler_S2k-8      74.9µs ± 2%    34.4µs ± 2%  -54.07%  (p=0.000 n=10+9)
GzipHandler_S20k-8      379µs ± 1%     226µs ± 3%  -40.42%  (p=0.000 n=9+10)
GzipHandler_S100k-8    1.95ms ± 2%    1.15ms ± 1%  -41.27%  (p=0.000 n=9+9)
GzipHandler_P2k-8      24.3µs ±25%    10.7µs ±25%  -55.80%  (p=0.000 n=10+10)
GzipHandler_P20k-8      132µs ± 2%      75µs ± 1%  -42.95%  (p=0.000 n=9+10)
GzipHandler_P100k-8     658µs ± 2%     371µs ± 3%  -43.68%  (p=0.000 n=9+10)

name                 old alloc/op   new alloc/op   delta
GzipHandler_S2k-8      7.71kB ± 5%    9.13kB ± 7%  +18.33%  (p=0.000 n=10+10)
GzipHandler_S20k-8     65.1kB ± 3%    70.3kB ± 3%   +8.05%  (p=0.000 n=10+10)
GzipHandler_S100k-8     348kB ± 4%     382kB ± 2%   +9.85%  (p=0.000 n=10+10)
GzipHandler_P2k-8      7.60kB ± 1%    7.93kB ± 2%   +4.33%  (p=0.000 n=10+10)
GzipHandler_P20k-8     64.4kB ± 1%    66.3kB ± 2%   +2.92%  (p=0.000 n=10+10)
GzipHandler_P100k-8     304kB ± 1%     309kB ± 1%   +1.67%  (p=0.000 n=10+9)

name                 old allocs/op  new allocs/op  delta
GzipHandler_S2k-8        21.0 ± 0%      21.0 ± 0%     ~     (all equal)
GzipHandler_S20k-8       24.0 ± 0%      24.0 ± 0%     ~     (all equal)
GzipHandler_S100k-8      27.0 ± 0%      27.0 ± 0%     ~     (all equal)
GzipHandler_P2k-8        21.0 ± 0%      21.0 ± 0%     ~     (all equal)
GzipHandler_P20k-8       24.0 ± 0%      24.0 ± 0%     ~     (all equal)
GzipHandler_P100k-8      26.0 ± 0%      26.0 ± 0%     ~     (all equal)
```

https://mattermost.atlassian.net/browse/MM-28491

go.mod

@@ -133,3 +133,5 @@ require (
 	honnef.co/go/tools v0.0.1-2020.1.3 // indirect
 	willnorris.com/go/imageproxy v0.10.0
 )
+
+replace github.com/NYTimes/gziphandler v1.1.1 => github.com/agnivade/gziphandler v1.1.2-0.20200815170021-7481835cb745

go.sum

@@ -26,8 +26,6 @@ github.com/Masterminds/semver v1.4.2/go.mod h1:MB6lktGJrhw8PrUyiEoblNEGEQ+RzHPF0
 github.com/Masterminds/squirrel v1.4.0 h1:he5i/EXixZxrBUWcxzDYMiju9WZ3ld/l7QBNuo/eN3w=
 github.com/Masterminds/squirrel v1.4.0/go.mod h1:yaPeOnPG5ZRwL9oKdTsO/prlkPbXWZlRVMQ/gGlzIuA=
 github.com/Masterminds/vcs v1.13.0/go.mod h1:N09YCmOQr6RLxC6UNHzuVwAdodYbbnycGHSmwVJjcKA=
-github.com/NYTimes/gziphandler v1.1.1 h1:ZUDjpQae29j0ryrS0u/B8HZfJBtBQHjqw2rQ2cqUQ3I=
-github.com/NYTimes/gziphandler v1.1.1/go.mod h1:n/CVRwUEOgIxrgPvAQhUUr9oeUtvrhMomdKFjzJNB0c=
 github.com/OneOfOne/xxhash v1.2.2/go.mod h1:HSdplMjZKSmBqAxg5vPj2TmRDmfkzw+cTzAElWljhcU=
 github.com/PaulARoy/azurestoragecache v0.0.0-20170906084534-3c249a3ba788/go.mod h1:lY1dZd8HBzJ10eqKERHn3CU59tfhzcAVb2c0ZhIWSOk=
 github.com/RoaringBitmap/roaring v0.4.21 h1:WJ/zIlNX4wQZ9x8Ey33O1UaD9TCTakYsdLFSBcTwH+8=
@@ -39,6 +37,8 @@ github.com/Shopify/sarama v1.19.0/go.mod h1:FVkBWblsNy7DGZRfXLU0O9RCGt5g3g3yEuWX
 github.com/Shopify/toxiproxy v2.1.4+incompatible/go.mod h1:OXgGpZ6Cli1/URJOF1DMxUHB2q5Ap20/P/eIdh4G0pI=
 github.com/VividCortex/gohistogram v1.0.0/go.mod h1:Pf5mBqqDxYaXu3hDrrU+w6nw50o/4+TcAqDqk/vUH7g=
 github.com/afex/hystrix-go v0.0.0-20180502004556-fa1af6a1f4f5/go.mod h1:SkGFH1ia65gfNATL8TAiHDNxPzPdmEL5uirI2Uyuz6c=
+github.com/agnivade/gziphandler v1.1.2-0.20200815170021-7481835cb745 h1:K5MCP8PQt50Tqkcdtgg7jhYwo6VBJWMKUmgqbXdu7WE=
+github.com/agnivade/gziphandler v1.1.2-0.20200815170021-7481835cb745/go.mod h1:eRIEzSVYGdp3PyjpBJPBPMo4LSeFUlpPjByDCzMRrtw=
 github.com/ajg/form v1.5.1/go.mod h1:uL1WgH+h2mgNtvBq0339dVnzXdBETtL2LeUXaIv25UY=
 github.com/alecthomas/template v0.0.0-20160405071501-a0175ee3bccc/go.mod h1:LOuyumcjzFXgccqObfd/Ljyb9UuFJ6TxHnclSeseNhc=
 github.com/alecthomas/template v0.0.0-20190718012654-fb15b899a751/go.mod h1:LOuyumcjzFXgccqObfd/Ljyb9UuFJ6TxHnclSeseNhc=
@@ -433,6 +433,8 @@ github.com/kisielk/errcheck v1.1.0/go.mod h1:EZBBE59ingxPouuu3KfxchcWSUPOHkagtvW
 github.com/kisielk/gotool v1.0.0/go.mod h1:XhKaO+MFFWcvkIS/tQcRk01m1F5IRFswLeQ+oQHNcck=
 github.com/klauspost/compress v1.8.2/go.mod h1:RyIbtBH6LamlWaDj8nUwkbUhJ87Yi3uG0guNDohfE1A=
 github.com/klauspost/compress v1.9.0/go.mod h1:RyIbtBH6LamlWaDj8nUwkbUhJ87Yi3uG0guNDohfE1A=
+github.com/klauspost/compress v1.10.11 h1:K9z59aO18Aywg2b/WSgBaUX99mHy2BES18Cr5lBKZHk=
+github.com/klauspost/compress v1.10.11/go.mod h1:aoV0uJVorq1K+umq18yTdKaF57EivdYsUV+/s2qKfXs=
 github.com/klauspost/cpuid v1.2.1/go.mod h1:Pj4uuM528wm8OyEC2QMXAi2YiTZ96dNQPGgoMS4s3ek=
 github.com/klauspost/cpuid v1.2.3 h1:CCtW0xUnWGVINKvE/WWOYKdsPV6mawAtvQuSl8guwQs=
 github.com/klauspost/cpuid v1.2.3/go.mod h1:Pj4uuM528wm8OyEC2QMXAi2YiTZ96dNQPGgoMS4s3ek=