Remove MaxIdleConns meaning no limit.
Set MaxIdleConnsPerHost to 512 as it must be limited, zero value implies the limit of 2 idle connections.

Fixes #665

net/http uses copyBufPool to copy body.

var copyBufPool = sync.Pool{
	New: func() any {
		b := make([]byte, 32*1024)
		return &b
	},
}

The default buffer size is 4KiB, increasing to 32KiB improves throughput by 5-10%.

Note the req/seq stability improvement to almost 100%.

$ wrk http://127.0.0.1:3128 -d 10 -t 1 -c 200
Running 10s test @ http://127.0.0.1:3128
  1 threads and 200 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     6.25ms    1.11ms  24.05ms   82.07%
    Req/Sec    32.15k     1.29k   34.36k    85.00%
  319722 requests in 10.01s, 30.49MB read
  Socket errors: connect 0, read 41, write 0, timeout 0
Requests/sec:  31944.24
Transfer/sec:      3.05MB

$ wrk http://127.0.0.1:3128 -d 10 -t 1 -c 200
Running 10s test @ http://127.0.0.1:3128
  1 threads and 200 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     5.74ms    1.24ms  33.95ms   89.38%
    Req/Sec    35.13k     1.37k   35.90k    99.00%
  349432 requests in 10.01s, 33.32MB read
  Socket errors: connect 0, read 41, write 0, timeout 0
Requests/sec:  34912.41
Transfer/sec:      3.33MB