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/hashicorp/go-plugin/plugin.go

@@ -9,6 +9,7 @@
 package plugin
 
 import (
+	"context"
 	"errors"
 	"net/rpc"
 
@@ -33,11 +34,12 @@ type GRPCPlugin interface {
 	// GRPCServer should register this plugin for serving with the
 	// given GRPCServer. Unlike Plugin.Server, this is only called once
 	// since gRPC plugins serve singletons.
-	GRPCServer(*grpc.Server) error
+	GRPCServer(*GRPCBroker, *grpc.Server) error
 
 	// GRPCClient should return the interface implementation for the plugin
-	// you're serving via gRPC.
-	GRPCClient(*grpc.ClientConn) (interface{}, error)
+	// you're serving via gRPC. The provided context will be canceled by
+	// go-plugin in the event of the plugin process exiting.
+	GRPCClient(context.Context, *GRPCBroker, *grpc.ClientConn) (interface{}, error)
 }
 
 // NetRPCUnsupportedPlugin implements Plugin but returns errors for the