Clearer logging of the account ID responsible for a blocking greenlet

[wojcikstefan]

When processing of an account blocks other greenlets, we log a warning about it here:

sync-engine/inbox/instrumentation.py

Lines 174 to 177 in e37f19a

	self.log.warning(
	'greenlet blocking', frame=formatted_frame,
	context=getattr(active_greenlet, 'context', None),
	blocking_greenlet_id=id(active_greenlet))

An example of a context logged is foldersyncengine:26:60 where 26 corresponds to the ID of the account being processed. I suggest that, instead of logging an enigmatic context, we should log an explicit account_id=context.split(':')[1] in this warning. I did NOT confirm that the format of the context is always the same though.