Database gRPC plugins (#3666)

* Start work on context aware backends

* Start work on moving the database plugins to gRPC in order to pass context

* Add context to builtin database plugins

* use byte slice instead of string

* Context all the things

* Move proto messages to the dbplugin package

* Add a grpc mechanism for running backend plugins

* Serve the GRPC plugin

* Add backwards compatibility to the database plugins

* Remove backend plugin changes

* Remove backend plugin changes

* Cleanup the transport implementations

* If grpc connection is in an unexpected state restart the plugin

* Fix tests

* Fix tests

* Remove context from the request object, replace it with context.TODO

* Add a test to verify netRPC plugins still work

* Remove unused mapstructure call

* Code review fixes

* Code review fixes

* Code review fixes

helper/pluginutil/runner.go

@@ -119,6 +119,10 @@ func (r *PluginRunner) runCommon(wrapper RunnerUtil, pluginMap map[string]plugin
 		SecureConfig:    secureConfig,
 		TLSConfig:       clientTLSConfig,
 		Logger:          namedLogger,
+		AllowedProtocols: []plugin.Protocol{
+			plugin.ProtocolNetRPC,
+			plugin.ProtocolGRPC,
+		},
 	}
 
 	client := plugin.NewClient(clientConfig)