Optimize gzip compression of responses (#15416)

It looks like maintainers aren't very active on the repo
and https://github.com/nytimes/gziphandler/pull/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

vendor/github.com/NYTimes/gziphandler/go.mod

@@ -2,4 +2,7 @@ module github.com/NYTimes/gziphandler
 
 go 1.11
 
-require github.com/stretchr/testify v1.3.0
+require (
+	github.com/klauspost/compress v1.10.11
+	github.com/stretchr/testify v1.3.0
+)

vendor/github.com/NYTimes/gziphandler/go.sum

@@ -1,5 +1,7 @@
 github.com/davecgh/go-spew v1.1.0 h1:ZDRjVQ15GmhC3fiQ8ni8+OwkZQO4DARzQgrnXU1Liz8=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+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/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=

vendor/github.com/NYTimes/gziphandler/gzip.go

@@ -2,7 +2,6 @@ package gziphandler // import "github.com/NYTimes/gziphandler"
 
 import (
 	"bufio"
-	"compress/gzip"
 	"fmt"
 	"io"
 	"mime"
@@ -11,6 +10,8 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+
+	"github.com/klauspost/compress/gzip"
 )
 
 const (
@@ -81,7 +82,7 @@ type GzipResponseWriter struct {
 
 	code int // Saves the WriteHeader value.
 
-	minSize int    // Specifed the minimum response size to gzip. If the response length is bigger than this value, it is compressed.
+	minSize int    // Specifies the minimum response size to gzip. If the response length is bigger than this value, it is compressed.
 	buf     []byte // Holds the first part of the write before reaching the minSize or the end of the write.
 	ignore  bool   // If true, then we immediately passthru writes to the underlying ResponseWriter.