Log line format

Log lines may contain the following fields:

u: user ID.
dev: device ID.
p: pos parameter from incoming requests.
q: pos value from outgoing responses.
t: transaction ID.
c: connection ID.
b: buffer summary.
Update counts in outgoing responses, each of which are omitted if zero:
- r: rooms.
- d: to-device events.
- ag: global account data updates.
- dl-c: changed devices.
- dl-l: left devices.
- sub: room subscriptions.
- usub: room unsubscriptions.
- l: lists.