* Sockets: fix Sockets.killWorker not disconnecting connections
* Sockets: fix unit tests
- Fix crash when constructing mock sockets in certain cases
- Properly prevent workers from writing to stdout
- Fix race conditions in workers-related tests that were causing false
positives
* Tests: mock workers now more closely imitate sockets' workers
This helps catch cases where messages are being sent in the wrong order
to the workers, e.g. messages sent to sockets that no longer exist.
If a child process dies in a way that it can't recover from, the main
process now exits, to prevent loops of 'Error: channel closed'
thousands of times a second, filling up the error log and making it
hard to track down the real error.
Specifically, Users.users, Users.connections, and Users.pastUsers
are now ES6 Maps. In theory, this should be a minor performance
upgrade, but we still need to profile to make sure.
