Move setting of Filter into code.

We do this to prevent foot guns. The default config uses a MemoryFilter,
but users are free to change to logging to files directly. If they do
then they have to ensure to set the `filters: [context]` on the right
handler, otherwise records get written with the wrong context.

Instead we move the logic to happen when we generate a record, which is
when we *log* rather than *handle*.

(It's possible to add filters to loggers in the config, however they
don't apply to descendant loggers and so they have to be manually set on
*every* logger used in the code base)

docker/conf/log.config

@@ -4,16 +4,10 @@ formatters:
   precise:
    format: '%(asctime)s - %(name)s - %(lineno)d - %(levelname)s - %(request)s - %(message)s'
 
-filters:
-  context:
-    (): synapse.logging.context.LoggingContextFilter
-    request: ""
-
 handlers:
   console:
     class: logging.StreamHandler
     formatter: precise
-    filters: [context]
 
 loggers:
     synapse.storage.SQL: