Improve S3 request signing performance

This change is caching HMAC hashers for repeated use in subsequent
requests and chunks, so they don't have to be initialized from
scratch every time.
On my local computer this gives me ~5-6 times faster signature
calculation and ~5-6.5% more throughput in S3 requests. The smaller
the payload the better the throughput gets.

weed/s3api/auth_credentials.go

@@ -2,12 +2,13 @@ package s3api
 
 import (
 	"fmt"
-	"github.com/seaweedfs/seaweedfs/weed/s3api/s3account"
 	"net/http"
 	"os"
 	"strings"
 	"sync"
 
+	"github.com/seaweedfs/seaweedfs/weed/s3api/s3account"
+
 	"github.com/seaweedfs/seaweedfs/weed/filer"
 	"github.com/seaweedfs/seaweedfs/weed/glog"
 	"github.com/seaweedfs/seaweedfs/weed/pb"
@@ -31,6 +32,8 @@ type IdentityAccessManagement struct {
 	identities    []*Identity
 	isAuthEnabled bool
 	domain        string
+	hashes        map[string]*sync.Pool
+	hashMu        sync.RWMutex
 }
 
 type Identity struct {
@@ -77,6 +80,7 @@ func (action Action) getPermission() Permission {
 func NewIdentityAccessManagement(option *S3ApiServerOption) *IdentityAccessManagement {
 	iam := &IdentityAccessManagement{
 		domain: option.DomainName,
+		hashes: make(map[string]*sync.Pool),
 	}
 	if option.Config != "" {
 		if err := iam.loadS3ApiConfigurationFromFile(option.Config); err != nil {