logging: Fix data race for sequence number.
logging: Fix data race for sequence number.

- Since we log from concurrent contexts, must atomically increment
  sequence number if we want it to be unique.
- This error was picked up by the race detector.

opentable/sous,opentable/sous,samsalisbury/sous,samsalisbury/sous,samsalisbury/sous,samsalisbury/sous,opentable/sous,samsalisbury/sous,opentable/sous,opentable/sous