Update plugin manager to properly flag shutdown is happening

[kaidaguerre]

Without this change, concurrent accesses to PluginManager.shuttingDown() will incorrectly report a shutdown is happening while there isn't.

Symptoms being, you see this in the logs:
2024-07-30 15:01:14.075 UTC [WARN] PluginManager Get failed for aws: plugin manager is shutting down (0xc009ba2450)
but there is no corresponding [INFO] PluginManager Shutdown