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.

synapse/crypto/keyring.py

@@ -43,8 +43,8 @@ from synapse.api.errors import (
     SynapseError,
 )
 from synapse.logging.context import (
-    LoggingContext,
     PreserveLoggingContext,
+    current_context,
     make_deferred_yieldable,
     preserve_fn,
     run_in_background,
@@ -236,7 +236,7 @@ class Keyring(object):
         """
 
         try:
-            ctx = LoggingContext.current_context()
+            ctx = current_context()
 
             # map from server name to a set of outstanding request ids
             server_to_request_ids = {}