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/crypto/test_keyring.py

@@ -34,6 +34,7 @@ from synapse.crypto.keyring import (
 from synapse.logging.context import (
     LoggingContext,
     PreserveLoggingContext,
+    current_context,
     make_deferred_yieldable,
 )
 from synapse.storage.keys import FetchKeyResult
@@ -83,9 +84,7 @@ class KeyringTestCase(unittest.HomeserverTestCase):
         )
 
     def check_context(self, _, expected):
-        self.assertEquals(
-            getattr(LoggingContext.current_context(), "request", None), expected
-        )
+        self.assertEquals(getattr(current_context(), "request", None), expected)
 
     def test_verify_json_objects_for_server_awaits_previous_requests(self):
         key1 = signedjson.key.generate_signing_key(1)
@@ -105,7 +104,7 @@ class KeyringTestCase(unittest.HomeserverTestCase):
 
         @defer.inlineCallbacks
         def get_perspectives(**kwargs):
-            self.assertEquals(LoggingContext.current_context().request, "11")
+            self.assertEquals(current_context().request, "11")
             with PreserveLoggingContext():
                 yield persp_deferred
             return persp_resp