Limit concurrency of async_get_integration to avoid creating extra threads (#43085)

* Limit concurrency of async_get_integration to avoid creating extra threads

Since async_get_integration is waiting on the disk most of the time
it would end up creating many new threads because the disk could
not deliver the data in time.

* pylint

homeassistant/loader.py

@@ -50,6 +50,8 @@ CUSTOM_WARNING = (
 )
 _UNDEF = object()
 
+MAX_LOAD_CONCURRENTLY = 4
+
 
 def manifest_from_legacy_module(domain: str, module: ModuleType) -> Dict:
     """Generate a manifest from a legacy module."""