Refactor logging in more Go idiomatic style (#6816)

This refactor brings a change which allows
targets to be added in a cleaner way and also
audit is now moved out.

This PR also simplifies logger dependency for auditing

cmd/logger/target/http/http.go

@@ -0,0 +1,94 @@
+/*
+ * Minio Cloud Storage, (C) 2018 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package http
+
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"io"
+	"io/ioutil"
+	gohttp "net/http"
+)
+
+// Target implements logger.Target and sends the json
+// format of a log entry to the configured http endpoint.
+// An internal buffer of logs is maintained but when the
+// buffer is full, new logs are just ignored and an error
+// is returned to the caller.
+type Target struct {
+	// Channel of log entries
+	logCh chan interface{}
+
+	// HTTP(s) endpoint
+	endpoint string
+	client   gohttp.Client
+}
+
+func (h *Target) startHTTPLogger() {
+	// Create a routine which sends json logs received
+	// from an internal channel.
+	go func() {
+		for entry := range h.logCh {
+			logJSON, err := json.Marshal(&entry)
+			if err != nil {
+				continue
+			}
+
+			req, err := gohttp.NewRequest("POST", h.endpoint, bytes.NewBuffer(logJSON))
+			req.Header.Set("Content-Type", "application/json")
+
+			resp, err := h.client.Do(req)
+			if err != nil {
+				continue
+			}
+			if resp.Body != nil {
+				buf := make([]byte, 512)
+				io.CopyBuffer(ioutil.Discard, resp.Body, buf)
+				resp.Body.Close()
+			}
+		}
+	}()
+}
+
+// New initializes a new logger target which
+// sends log over http to the specified endpoint
+func New(endpoint string, transport *gohttp.Transport) *Target {
+	h := Target{
+		endpoint: endpoint,
+		client: gohttp.Client{
+			Transport: transport,
+		},
+		logCh: make(chan interface{}, 10000),
+	}
+
+	h.startHTTPLogger()
+	return &h
+}
+
+// Send log message 'e' to http target.
+func (h *Target) Send(entry interface{}) error {
+	select {
+	case h.logCh <- entry:
+	default:
+		// log channel is full, do not wait and return
+		// an error immediately to the caller
+		return errors.New("log buffer full")
+	}
+
+	return nil
+}