Add new httpSink

This sink is compatible with the HTTP API of cc-metric-store.
Example config.json section:

```
"sink": {
    "type": "http",
    "host": "localhost",
    "port": "8080",
    "database": "api/write",
    "password": "<JWT>"
},
```

The password/JWT can be omitted.

sinks/httpSink.go

@@ -0,0 +1,68 @@
+package sinks
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"net/http"
+	"time"
+
+	lp "github.com/influxdata/line-protocol"
+)
+
+type HttpSink struct {
+	Sink
+	client   *http.Client
+	url, jwt string
+	encoder  *lp.Encoder
+	buffer   *bytes.Buffer
+}
+
+func (s *HttpSink) Init(config SinkConfig) error {
+	if len(config.Host) == 0 || len(config.Port) == 0 {
+		return errors.New("`host`, `port` and `database` config options required for TCP sink")
+	}
+
+	s.client = &http.Client{}
+	s.url = fmt.Sprintf("http://%s:%s/%s", config.Host, config.Port, config.Database)
+	s.port = config.Port
+	s.jwt = config.Password
+	s.buffer = &bytes.Buffer{}
+	s.encoder = lp.NewEncoder(s.buffer)
+	s.encoder.SetPrecision(time.Second)
+
+	return nil
+}
+
+func (s *HttpSink) Write(point lp.MutableMetric) error {
+	_, err := s.encoder.Encode(point)
+	return err
+}
+
+func (s *HttpSink) Flush() error {
+	req, err := http.NewRequest(http.MethodPost, s.url, s.buffer)
+	if err != nil {
+		return err
+	}
+
+	if len(s.jwt) != 0 {
+		req.Header.Set("Authorization", fmt.Sprintf("Bearer %s", s.jwt))
+	}
+
+	res, err := s.client.Do(req)
+	s.buffer.Reset()
+
+	if err != nil {
+		return err
+	}
+
+	if res.StatusCode != 200 {
+		return errors.New(res.Status)
+	}
+
+	return nil
+}
+
+func (s *HttpSink) Close() {
+	s.client.CloseIdleConnections()
+}