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/http/federation/test_matrix_federation_agent.py

@@ -38,7 +38,7 @@ from synapse.http.federation.well_known_resolver import (
     WellKnownResolver,
     _cache_period_from_headers,
 )
-from synapse.logging.context import LoggingContext
+from synapse.logging.context import SENTINEL_CONTEXT, LoggingContext, current_context
 from synapse.util.caches.ttlcache import TTLCache
 
 from tests import unittest
@@ -155,7 +155,7 @@ class MatrixFederationAgentTests(unittest.TestCase):
             self.assertNoResult(fetch_d)
 
             # should have reset logcontext to the sentinel
-            _check_logcontext(LoggingContext.sentinel)
+            _check_logcontext(SENTINEL_CONTEXT)
 
             try:
                 fetch_res = yield fetch_d
@@ -1197,7 +1197,7 @@ class TestCachePeriodFromHeaders(unittest.TestCase):
 
 
 def _check_logcontext(context):
-    current = LoggingContext.current_context()
+    current = current_context()
     if current is not context:
         raise AssertionError("Expected logcontext %s but was %s" % (context, current))