groot/internal/httpio: add a pool of http.Request

This CL adds a pool of http.Request to alleviate somewhat the memory
pressure of cloning those when requesting new chunks of data from the
HTTP server.

old: reading remote file after having downloaded to a local one
new: reading remote file w/ httpio + sync.Pool

```
  name                    old time/op    new time/op    delta
  DumpHTTP/delay-0s-8        279ms ± 1%     287ms ± 2%   +3.09%  (p=0.000 n=27+29)
  DumpHTTP/delay-5ms-8       279ms ± 2%     288ms ± 2%   +3.06%  (p=0.000 n=29+29)
  DumpHTTP/delay-10ms-8      281ms ± 3%     287ms ± 1%   +2.24%  (p=0.000 n=26+30)
  DumpHTTP/delay-100ms-8     286ms ± 3%     288ms ± 1%   +0.45%  (p=0.032 n=25+29)

  name                    old alloc/op   new alloc/op   delta
  DumpHTTP/delay-0s-8        156MB ± 0%     159MB ± 0%   +1.93%  (p=0.000 n=29+30)
  DumpHTTP/delay-5ms-8       156MB ± 0%     159MB ± 0%   +1.95%  (p=0.000 n=30+30)
  DumpHTTP/delay-10ms-8      155MB ± 0%     159MB ± 0%   +1.95%  (p=0.000 n=30+30)
  DumpHTTP/delay-100ms-8     155MB ± 0%     159MB ± 0%   +1.99%  (p=0.000 n=30+28)

  name                    old allocs/op  new allocs/op  delta
  DumpHTTP/delay-0s-8        81.4k ± 0%    117.6k ± 0%  +44.46%  (p=0.000 n=30+30)
  DumpHTTP/delay-5ms-8       81.4k ± 0%    117.6k ± 0%  +44.48%  (p=0.000 n=30+30)
  DumpHTTP/delay-10ms-8      81.4k ± 0%    117.5k ± 0%  +44.48%  (p=0.000 n=29+30)
  DumpHTTP/delay-100ms-8     81.4k ± 0%    117.6k ± 0%  +44.51%  (p=0.000 n=30+29)
```

Fixes #142.

groot/internal/httpio/reader.go

@@ -10,6 +10,7 @@ import (
 	"io"
 	"net/http"
 	"strconv"
+	"sync"
 )
 
 // Reader presents an HTTP resource as an io.Reader and io.ReaderAt.
@@ -19,6 +20,8 @@ type Reader struct {
 	ctx    context.Context
 	cancel context.CancelFunc
 
+	pool sync.Pool
+
 	r    *io.SectionReader
 	len  int64
 	etag string
@@ -69,6 +72,13 @@ func Open(uri string, opts ...Option) (r *Reader, err error) {
 	r.etag = hdr.Header.Get("Etag")
 	r.r = io.NewSectionReader(r, 0, r.len)
 
+	r.req.Header.Set("Range", "")
+	r.pool = sync.Pool{
+		New: func() interface{} {
+			return r.req.Clone(r.ctx)
+		},
+	}
+
 	return r, nil
 }
 
@@ -107,8 +117,8 @@ func (r *Reader) ReadAt(p []byte, off int64) (int, error) {
 	}
 
 	rng := rng(off, off+int64(len(p))-1)
-	req := r.req.Clone(r.ctx)
-	req.Header.Set("Range", rng)
+	req := r.getReq(rng)
+	defer r.pool.Put(req)
 
 	resp, err := r.cli.Do(req)
 	if err != nil {
@@ -138,6 +148,13 @@ func (r *Reader) ReadAt(p []byte, off int64) (int, error) {
 	return n, nil
 }
 
+func (r *Reader) getReq(rng string) *http.Request {
+	o := r.pool.Get().(*http.Request)
+	o.Header = r.req.Header.Clone()
+	o.Header["Range"][0] = rng
+	return o
+}
+
 func rng(beg, end int64) string {
 	return "bytes=" + strconv.Itoa(int(beg)) + "-" + strconv.Itoa(int(end))
 }