resolver: disable http2 for pushing
resolver: disable http2 for pushing

The golang net/http package uses http2 client to serve https by default,
if let Transport.TLSNextProto is nil. And net/http package doesn't
provide tunnable value for http2 flow control which will limit push
performance.

Before this commit, use GODEBUG="http2debug=1" buildkitd to pushing
one image from dockerfile like

```
$ about 700MB
FROM scratch
ADD ./golang-1.13.0-stretch.tar.gzip /
```

and use ifstat to monitor network interface and found that

```
$ ifstat -i enp0s3
      enp0s3
 KB/s in  KB/s out
    0.47      0.67
    0.44      0.51
   19.72     11.56
   62.25   2184.41
   96.34   3514.28
   93.89   3508.31
   95.41   3515.53
   91.61   3433.22
   95.82   3579.68
   90.36   3388.89
   93.64   3513.03
   93.32   3478.04
   ...

$ log from buildkitd
2020/03/31 17:40:33 http2: Transport received WINDOW_UPDATE stream=11 len=4 incr=32768
2020/03/31 17:40:33 http2: Transport received WINDOW_UPDATE len=4 (conn) incr=271
2020/03/31 17:40:33 http2: Transport received WINDOW_UPDATE stream=11 len=4 incr=271
2020/03/31 17:40:33 http2: Transport received WINDOW_UPDATE len=4 (conn) incr=32496
2020/03/31 17:40:33 http2: Transport received WINDOW_UPDATE stream=11 len=4 incr=32496
...
```

The registry will update window size of flow control when receives each
frame data. The sender need wait for receiver update the window size if
the sender runs out of buffer of flow control. But the increase value
for buffer by WINDOW_UPDATE frame is too small and slow which impacts
push performance.

Before net/http package provides tunnable value for flow control, we
should disable http2 for https request.

And with this commit, the performance will be better like:

```
$ ifstat -i enp0s3
      enp0s3
 KB/s in  KB/s out
    0.56      0.61
   16.13      5.55
   18.89      9.23
  218.84   7832.80
  338.56  13074.04
  302.39  11713.83
  231.62   8964.60
  356.50  13504.02
  298.14  11401.81
  311.24  11783.26
  333.01  12710.17
  329.64  12630.40
  305.87  11662.04
  292.53  11118.04
```

Signed-off-by: Wei Fu <df4176d36abdc6fd0d966de5d5471522668de494@gmail.com>

tonistiigi/buildkit_poc,tonistiigi/buildkit_poc