optimized memory alloc

[kazeburo]

reduce memory alloc in header copy.
I saw golang/go#29915.

use io.copybuffer & sync.pool to response.

[kazeburo]

new

Concurrency Level: 10
Time taken for tests: 0.691 seconds
Complete requests: 10000
Failed requests: 0
Keep-Alive requests: 10000
Total transferred: 8869779 bytes
HTML transferred: 6120000 bytes
Requests per second: 14471.59 [#/sec] (mean)
Time per request: 0.691 [ms] (mean)
Time per request: 0.069 [ms] (mean, across all concurrent requests)
Transfer rate: 12535.14 [Kbytes/sec] received

current

Concurrency Level: 10
Time taken for tests: 0.765 seconds
Complete requests: 10000
Failed requests: 0
Keep-Alive requests: 10000
Total transferred: 8869768 bytes
HTML transferred: 6120000 bytes
Requests per second: 13070.44 [#/sec] (mean)
Time per request: 0.765 [ms] (mean)
Time per request: 0.077 [ms] (mean, across all concurrent requests)
Transfer rate: 11321.46 [Kbytes/sec] received