Use ReadFull instead of ioutil.ReadAll to read objects (#76)

* Use ReadFull instead of ioutil.ReadAll to read objects

ioutil.ReadAll uses a bytes.Buffer, which allocates memory by doubling.
Since we know exactly how muh data we expect to get, we can allocate it
in advance. This reduces the total amount of allocation, and ensures
that the slice stored in the item won't have excess capacity (which can
affect memory usage if the item is held for a long time, for instance in
a secondary cache).

memcache/memcache.go

@@ -23,7 +23,6 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"net"
 
 	"strconv"
@@ -481,11 +480,14 @@ func parseGetResponse(r *bufio.Reader, cb func(*Item)) error {
 		if err != nil {
 			return err
 		}
-		it.Value, err = ioutil.ReadAll(io.LimitReader(r, int64(size)+2))
+		it.Value = make([]byte, size+2)
+		_, err = io.ReadFull(r, it.Value)
 		if err != nil {
+			it.Value = nil
 			return err
 		}
 		if !bytes.HasSuffix(it.Value, crlf) {
+			it.Value = nil
 			return fmt.Errorf("memcache: corrupt get result read")
 		}
 		it.Value = it.Value[:size]