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

1 files changed, 4 insertions, 2 deletions

diff --git a/app/app.go b/app/app.go
index 2cdf333c1..6de75855c 100644
--- a/app/app.go
+++ b/app/app.go
@@ -38,8 +38,10 @@ type App struct {
 
 	Log *mlog.Logger
 
-	PluginEnv              *pluginenv.Environment
-	PluginConfigListenerId string
+	PluginEnv                *pluginenv.Environment
+	PluginConfigListenerId   string
+	IsPluginSandboxSupported bool
+	pluginStatuses           map[string]*model.PluginStatus
 
 	EmailBatching *EmailBatchingJob