Fix HTTP keep-alive for CouchDB connections (#4497)

Golang can reuse HTTP connections with keep alive, but the response body
must have been fully read and closed. In the couchdb package, we were
reading the body for sucessful response with a json.Decoder. It only
reads a JSON value, and most responses from CouchDB have an additional
\n after that JSON value. This last byte was not read from the response
body, and it prevents Golang from reusing the connection. So, we are
doing a new HTTP connection (with DNS resolution and TLS handshake) for
each request to CouchDB.

pkg/couchdb/couchdb.go

@@ -311,7 +311,11 @@ func makeRequest(db prefixer.Prefixer, doctype, method, path string, reqbody int
 		log.Error(err.Error())
 		return err
 	}
-	defer resp.Body.Close()
+	defer func() {
+		// Flush the body, so that the connection can be reused by keep-alive
+		_, _ = io.Copy(io.Discard, resp.Body)
+		_ = resp.Body.Close()
+	}()
 
 	if elapsed.Seconds() >= 10 {
 		log.Infof("slow request on %s %s (%s)", method, path, elapsed)
@@ -322,8 +326,6 @@ func makeRequest(db prefixer.Prefixer, doctype, method, path string, reqbody int
 		return err
 	}
 	if resbody == nil {
-		// Flush the body, so that the connection can be reused by keep-alive
-		_, _ = io.Copy(io.Discard, resp.Body)
 		return nil
 	}