MM-8622: Improved plugin error reporting (#8737)

* allow `Wait()`ing on the supervisor

In the event the plugin supervisor shuts down a plugin for crashing too
many times, the new `Wait()` interface allows the `ActivatePlugin` to
accept a callback function to trigger when `supervisor.Wait()` returns.
If the supervisor shuts down normally, this callback is invoked with
a nil error, otherwise any error reported by the supervisor is passed
along.

* improve plugin activation/deactivation logic

Avoid triggering activation of previously failed-to-start plugins just
becase something in the configuration changed. Now, intelligently
compare the global enable bit as well as the each individual plugin's
enabled bit.

* expose store to manipulate PluginStatuses

* expose API to fetch plugin statuses

* keep track of whether or not plugin sandboxing is supported

* transition plugin statuses

* restore error on plugin activation if already active

* don't initialize test plugins until successfully loaded

* emit websocket events when plugin statuses change

* skip pruning if already initialized

* MM-8622: maintain plugin statuses in memory

Switch away from persisting plugin statuses to the database, and
maintain in memory instead. This will be followed by a cluster interface
to query the in-memory status of plugin statuses from all cluster nodes.

At the same time, rename `cluster_discovery_id` on the `PluginStatus`
model object to `cluster_id`.

* MM-8622: aggregate plugin statuses across cluster

* fetch cluster plugin statuses when emitting websocket notification

* address unit test fixes after rebasing

* relax (poor) racey unit test re: supervisor.Wait()

* make store-mocks

plugin/pluginenv/environment.go

@@ -108,7 +108,7 @@ func (env *Environment) IsPluginActive(pluginId string) bool {
 }
 
 // Activates the plugin with the given id.
-func (env *Environment) ActivatePlugin(id string) error {
+func (env *Environment) ActivatePlugin(id string, onError func(error)) error {
 	env.mutex.Lock()
 	defer env.mutex.Unlock()
 
@@ -117,7 +117,7 @@ func (env *Environment) ActivatePlugin(id string) error {
 	}
 
 	if _, ok := env.activePlugins[id]; ok {
-		return nil
+		return fmt.Errorf("plugin already active: %v", id)
 	}
 	plugins, err := ScanSearchPath(env.searchPath)
 	if err != nil {
@@ -156,6 +156,14 @@ func (env *Environment) ActivatePlugin(id string) error {
 		if err := supervisor.Start(api); err != nil {
 			return errors.Wrapf(err, "unable to start plugin: %v", id)
 		}
+		if onError != nil {
+			go func() {
+				err := supervisor.Wait()
+				if err != nil {
+					onError(err)
+				}
+			}()
+		}
 
 		activePlugin.Supervisor = supervisor
 	}