Clean up some LoggingContext stuff (#7120)

* Pull Sentinel out of LoggingContext

... and drop a few unnecessary references to it

* Factor out LoggingContext.current_context

move `current_context` and `set_context` out to top-level functions.

Mostly this means that I can more easily trace what's actually referring to
LoggingContext, but I think it's generally neater.

* move copy-to-parent into `stop`

this really just makes `start` and `stop` more symetric. It also means that it
behaves correctly if you manually `set_log_context` rather than using the
context manager.

* Replace `LoggingContext.alive` with `finished`

Turn `alive` into `finished` and make it a bit better defined.

tests/utils.py

@@ -35,7 +35,7 @@ from synapse.config.homeserver import HomeServerConfig
 from synapse.config.server import DEFAULT_ROOM_VERSION
 from synapse.federation.transport import server as federation_server
 from synapse.http.server import HttpServer
-from synapse.logging.context import LoggingContext
+from synapse.logging.context import current_context, set_current_context
 from synapse.server import HomeServer
 from synapse.storage import DataStore
 from synapse.storage.engines import PostgresEngine, create_engine
@@ -493,10 +493,10 @@ class MockClock(object):
         return self.time() * 1000
 
     def call_later(self, delay, callback, *args, **kwargs):
-        current_context = LoggingContext.current_context()
+        ctx = current_context()
 
         def wrapped_callback():
-            LoggingContext.thread_local.current_context = current_context
+            set_current_context(ctx)
             callback(*args, **kwargs)
 
         t = [self.now + delay, wrapped_callback, False]