pkg/canary: use default HTTP client when reading from Loki

The documentation[1] for net/http.Client says the following:

  A Client is an HTTP client. Its zero value (DefaultClient) is a usable
  client that uses DefaultTransport.

  The Client's Transport typically has internal state (cached TCP
  connections), so Clients should be reused instead of created as needed.
  Clients are safe for concurrent use by multiple goroutines.

  A Client is higher-level than a RoundTripper (such as Transport) and
  additionally handles HTTP details such as cookies and redirects.

Canary was creating a new http.Client every time a query request was
made to Loki, causing a new TCP connection to be established each time.
If Loki has an outage, it's possible to get into a situation where all
websocket requests are failing and for a high volume of requests to go
through the normal query route. This can possibly lead to networking
issues like socket starvation.

[1]: https://golang.org/pkg/net/http/#Client

pkg/canary/reader/reader.go

@@ -109,16 +109,14 @@ func (r *Reader) Query(start time.Time, end time.Time) ([]time.Time, error) {
 	}
 	_, _ = fmt.Fprintf(r.w, "Querying loki for missing values with query: %v\n", u.String())
 
-	client := &http.Client{}
-
 	req, err := http.NewRequest("GET", u.String(), nil)
 	if err != nil {
 		return nil, err
 	}
 
 	req.SetBasicAuth(r.user, r.pass)
 
-	resp, err := client.Do(req)
+	resp, err := http.DefaultClient.Do(req)
 	if err != nil {
 		return nil, err
 	}