* 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
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