Backend image rendering as plugin (#11966)

* rendering: headless chrome progress

* renderer: minor change

* grpc: version hell

* updated grpc libs

* wip: minor progess

* rendering: new image rendering plugin is starting to work

* feat: now phantomjs works as well and updated alerting to use new rendering service

* refactor: renamed renderer package and service to rendering to make renderer name less confusing (rendering is internal service that handles the renderer plugin now)

* rendering: now render key is passed and render auth is working in plugin mode

* removed unneeded lines from gitignore

* rendering: now plugin mode supports waiting for all panels to complete rendering

* fix: LastSeenAt fix for render calls, was not set which causes a lot of updates to Last Seen at during rendering, this should fix sqlite db locked issues in seen in previous releases

* change: changed render tz url parameter to use proper timezone name as chrome does not handle UTC offset TZ values

* fix: another update to tz param generation

* renderer: added http mode to renderer service, new ini setting [rendering] server_url

vendor/github.com/uber/jaeger-client-go/log/logger.go

@@ -47,7 +47,7 @@ func (l *stdLogger) Infof(msg string, args ...interface{}) {
 	log.Printf(msg, args...)
 }
 
-// NullLogger is implementation of the Logger interface that delegates to default `log` package
+// NullLogger is implementation of the Logger interface that is no-op
 var NullLogger = &nullLogger{}
 
 type nullLogger struct{}
@@ -81,3 +81,10 @@ func (l *BytesBufferLogger) String() string {
 	defer l.mux.Unlock()
 	return l.buf.String()
 }
+
+// Flush empties the underlying buffer.
+func (l *BytesBufferLogger) Flush() {
+	l.mux.Lock()
+	defer l.mux.Unlock()
+	l.buf.Reset()
+}