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/federation/federation_base.py

@@ -32,8 +32,8 @@ from synapse.events import EventBase, make_event_from_dict
 from synapse.events.utils import prune_event
 from synapse.http.servlet import assert_params_in_dict
 from synapse.logging.context import (
-    LoggingContext,
     PreserveLoggingContext,
+    current_context,
     make_deferred_yieldable,
 )
 from synapse.types import JsonDict, get_domain_from_id
@@ -78,7 +78,7 @@ class FederationBase(object):
         """
         deferreds = _check_sigs_on_pdus(self.keyring, room_version, pdus)
 
-        ctx = LoggingContext.current_context()
+        ctx = current_context()
 
         def callback(_, pdu: EventBase):
             with PreserveLoggingContext(ctx):