Add audit logging for S3 and Web handlers (#6571)

This PR brings an additional logger implementation
called AuditLog which logs to http targets

The intention is to use AuditLog to log all incoming
requests, this is used as a mechanism by external log
collection entities for processing Minio requests.

cmd/acl-handlers.go

@@ -21,6 +21,7 @@ import (
 	"net/http"
 
 	"github.com/gorilla/mux"
+	"github.com/minio/minio/cmd/logger"
 	"github.com/minio/minio/pkg/policy"
 )
 
@@ -56,6 +57,8 @@ type accessControlPolicy struct {
 func (api objectAPIHandlers) GetBucketACLHandler(w http.ResponseWriter, r *http.Request) {
 	ctx := newContext(r, w, "GetBucketACL")
 
+	defer logger.AuditLog(ctx, r)
+
 	vars := mux.Vars(r)
 	bucket := vars["bucket"]
 
@@ -103,6 +106,8 @@ func (api objectAPIHandlers) GetBucketACLHandler(w http.ResponseWriter, r *http.
 func (api objectAPIHandlers) GetObjectACLHandler(w http.ResponseWriter, r *http.Request) {
 	ctx := newContext(r, w, "GetObjectACL")
 
+	defer logger.AuditLog(ctx, r)
+
 	vars := mux.Vars(r)
 	bucket := vars["bucket"]
 	object := vars["object"]