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/storage/data_stores/main/events_worker.py

@@ -35,7 +35,7 @@ from synapse.api.room_versions import (
 )
 from synapse.events import make_event_from_dict
 from synapse.events.utils import prune_event
-from synapse.logging.context import LoggingContext, PreserveLoggingContext
+from synapse.logging.context import PreserveLoggingContext, current_context
 from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.storage._base import SQLBaseStore, make_in_list_sql_clause
 from synapse.storage.database import Database
@@ -409,7 +409,7 @@ class EventsWorkerStore(SQLBaseStore):
         missing_events_ids = [e for e in event_ids if e not in event_entry_map]
 
         if missing_events_ids:
-            log_ctx = LoggingContext.current_context()
+            log_ctx = current_context()
             log_ctx.record_event_fetch(len(missing_events_ids))
 
             # Note that _get_events_from_db is also responsible for turning db rows