- allows us to generate session-specific ids

- fixes a problem when running with AutobahnPython and twisted

- prevent data race; maps aren't safe for concurrent use

- fixes problems with multiple subscribers to the same topic

- no guarantee they still work, but this makes it easier to run tests

- single rw lock to make eliminate races, could definitely use some
  performance tuning/refactoring

- it seems like this was blocking when the socket was already closed,
  which caused some stability issues over time as goroutines built up

- I noticed a that defaultBroker.Subscribe could block, and the only way
  that could happen is if the Send function in either Unsubscribe
  (unlikely) or Publish (very likely) blocked
- I think there's still a larger problem with transports blocking on the
  Send, but this should at least allow messages to go through if that
  happens

Also added a write timeout.

- fixes memory leak

This should probably be easier to set.